This refactors template management to get logic out of DefaultGenerator
and to provide a cleaner API to template search and read/compile.

Deprecates MockDefaultGenerator, which is not a mock and causes
in-memory retention of file contents. Maintainers should prefer
executing a "dryRun" with new DefaultGenerator(true) or do true
mock/spies if evaluating template intermediaries is truly necessary.
Tests may read written files with lower overhead than the in-process
retention of those bytes.

This attempts to maintain some compatibility with existing templating
adapter interfaces. Any breaking change here would be unintentional but
minimal effort to retarget the new interface.

…anagerTest)

The samples scripts for Java incorrectly referenced the libraries
directories directly rather than the upper-level Java directory. This
was incorrect usage of template directories, because the generator
expects to be given the "language" directory and perform a lookup for
missing templates in the order:

* user defined libraries directory
* user defined language root
* embedded libraries directory
* embedded language root
* _common directory

This is incorrect in our samples scripts because a user or maintainer
has the expectation that any template change to files at the Java/ root
should also be honored on generation if the script specifies a custom
template directory.

* master:
  [samples] Regenerate python-experimental
  [core][general] Add metadata file tracking to aid in "Golden Tests" regeneration (OpenAPITools#6325)
  [python-experimental] Add support for pep 3134, attach cause of exception (OpenAPITools#6388)
  [Java-jersey2] Add new ApiClient constructor with auth objects (OpenAPITools#6393)

HandlebarseEngineAdapter previously didn't handle files without
extensions in the same was as the MustacheEngineAdapter. This now allows
for files without extension (or dotfiles) to lookup in the same
location.

Meta tasks are cleaned up to use template manager only, rather than
attempting to create an "empty" generator to use the previous templating
specific methods.

* master:
  update java jersey2 samples
  [Java] Fix mustache tag in pom template for HTTP signature (OpenAPITools#6404)
  [Python-experimental] Rename from_server variable to json_variable_naming (OpenAPITools#6390)
  Add a link to medium blog post (OpenAPITools#6403)
  Clean up debug in test (OpenAPITools#6398)
  readding bin/swift5-petstore-readonlyProperties.json
  remove ./bin/swift5-petstore-readonlyProperties.json

* master:
  decomission nodejs server generator (OpenAPITools#6406)
  [Java] Generate valid code if no Authentication implementations present (OpenAPITools#5788)

* master:
  [java-jersey2] Conditionally include http signature mustache template (OpenAPITools#6413)
  [bug] Fix path provider bug on CI (OpenAPITools#6409)

The .gitignore file is unable to load via classpath resource from the
graphql node server resource directory (for unknown reasons). Before
this change, the missing template would fail silently.

A .gitignore file may exist in other directories and load as expected.
Added a default .gitignore to _common as a fallback so as not to break
any custom generators which may also be failing silently.

Java resources may not load .gitignore, this follows suit with other
generators and uses "gitignore" (some use "gitignore.mustache").

* master:
  [kotlin][client] add support for coroutines with OkHttp (OpenAPITools#6362)
  update package-json.lock (OpenAPITools#6430)
  fix hardcoded match type (OpenAPITools#6431)
  java jersey2 enhance anyOf (OpenAPITools#6420)
  [Java][jersey2] minor improvement to jersey2 tests (OpenAPITools#6418)
  ps minor style change (OpenAPITools#6424)
  [JS] mark ES5 as deprecated (OpenAPITools#6408)
  [ci] Execute maven and verify with no-snapshot-updates (OpenAPITools#6415)
  add new file in ts axios samples
  Migrate all scala generators to use OAS3 (OpenAPITools#6407)
  migrate ruby samples to oas3 (OpenAPITools#6414)
  [PS] check if JSON properties is defined (OpenAPITools#6419)
  Add C++ UE4  client generator (OpenAPITools#6399)
  Add a link to the article in dev.to (OpenAPITools#6421)
  typescript-axios anytype is not defined (OpenAPITools#6335)
  [Java][jersey2] Make (de)serialization work for oneOf models, add convenience and comparison methods (OpenAPITools#6323)
  Migrate OCaml petstore to use OAS v3 spec (OpenAPITools#6348)
  [Python-experimental] Fix type error if oneof/anyof child schema is null type (OpenAPITools#6387)
  [Python-server] Fix blueplanet 'file not found' error  (OpenAPITools#6411)
  [nodejs] Fix deprecation notice when running sample nodejs script (OpenAPITools#6412)

* master:
  Fix ruby deprecation error (OpenAPITools#6450)
  [Java][Feign] decommission 9.x support (OpenAPITools#6445)
  fix struct export in rust reqwest (OpenAPITools#6453)
  [Rust][reqwest] add tests to CI (OpenAPITools#6454)
  set pester version (OpenAPITools#6448)
  update datadog logo
  update groovy petstore samples
  add datadog as sponsor (OpenAPITools#6444)
  Migrate Groovy samples to oas3 (OpenAPITools#6435)
  [samples] Renerate Kotlin coroutines

* master:
  [PS] Refactor the http signing auth with ecdsa support (OpenAPITools#6397)
  [Rust Server] Hyper 0.13 + Async/Await support (OpenAPITools#6244)
  [Rust] set supportAsync to true as the default (OpenAPITools#6480)
  [php-symfony] Set required PHP version ^7.1.3 (OpenAPITools#6181)
  update doc
  [csharp] Rename netstandard to netstandard1.3 (OpenAPITools#6460)
  feat: support deprecated parameters for typescript-axios generator (OpenAPITools#6475)
  [REQ][typescript-axios] useSingleRequestParameter should mark parameter optional if all properties are optional (OpenAPITools#6477)
  better struct alias in rust (OpenAPITools#6470)
  Migrate Go server samples to OAS 3 only (OpenAPITools#6471)
  [Rust][reqwest] add async support (OpenAPITools#6464)
  [codegen][python-experimental] Composed schema with additionalProperties  (OpenAPITools#6290)
  [Java] Decommission Retrofit 1.x support (OpenAPITools#6447)
  remove scala oas3 samples (OpenAPITools#6446)
  [Java][jersey2] Fix RuntimeException when HTTP signature parameters are not configured (OpenAPITools#6457)
  [Java][jersey2] Improve http signature code comments (OpenAPITools#6463)
  [typescript-angular] drop support of angular below 6.0.0 (OpenAPITools#6360)
  [cli] new 'author template' command (OpenAPITools#6441)
  python-experimental updates ancestor + adds descendant discriminator tests (OpenAPITools#6417)