Static content support module.

[tomas-langer]

Resolves #1869
Resolves #1236
Resolves #2631
Resolves #2764

There is a new module: helidon-webserver-static-content.
Entry point is io.helidon.webserver.staticcontent.StaticContentSupport.builder(..)

To achieve a fallback for static content, you can use the following when setting up routing:

Routing.builder()
  .register("/static", StaticContentSupport.builder("web")
    .welcomeFileName("index.txt")
    .build())
  .register("/static", StaticContentSupport.builder("fallback")
    .pathMapper(path -> "index.txt")
    .build())

This will resolve any request to /static as follows:

1. Attempt to find a resource on the classpath under web, using index.txt as the welcome file for directory requests
2. Use fallback/index.txt for any request that does not find a resource in the first step

The default pathMapper does not modify the path (so for static/css/company.css it returns static/css/company.css).

There is a known duplicate use of temporary directory - each instance of static content support based on classpath would extract the files separately. If we used the same classpath location to read the "fallback" file, it would be extracted and cached twice.
As this use-case is based on the need to have a single backup file, the impact is minimal (extracting one extra file). If this is a major performance issue for your application, please submit a new issue with your use case.

[romain-grecourt]

Shouldn't this PR be targeted for 3.x ? It breaks compatibility right ?

[tomas-langer]

I have only created a new module and deprecated the old classes. Everything still works as it used to.

[tomas-langer]

Will need to incorporate changes from #2686 if we decide to merge them

[tomas-langer]

Check #2631

We should support a fallback to a single file from static content.

Request to /static/webserver/index.html - which does not exist, should return /static/index.html.

This would allow for routing like this:

Routing.builder()
  .register("/static", StaticContentSupport.create("/static")
 .register("/static", SingleFileContent.create("/index.html")

[tomas-langer]

Refactored implementation to have nicer builders, added integration tests to validate implementation.

[zimmi]

Maybe a minor thing, but fallback/index.html will be extracted twice to the tmp directory when using two separate instances.

[tomas-langer]

I do not think this is the case. There is one based on classpath, one based on path.

[romain-grecourt]

This is a side-effect of the current implementation of the classpath based StaticContentSupport.