Create proto_toolchain rule

[Yannic]

This change adds a new proto_toolchain rule which we will use later to resolve
tools and options required to compile Protocol Buffers, thus eventually replacing
some of the command-line options from ProtoConfiguration.

To preclude inconsistencies in the migration period, we will start with a
proto_toolchain rule that doesn't have any arguments and populate
its provider from the existing command-line options. After all consumers
have been migrated to resolve the tools from the toolchain instead of
reading the command-line, we will replace all proto-specific flags with
attributes on proto_toolchain (or similar, language-specific toolchains
for flags that only affect a specific language).

This change also adds a dependency on the new toolchain type to
proto_library, cc_proto_library, java_proto_library, and java_lite_proto_library.
Unfortunately, there seems to be no existing way to add a dependency on a
toolchain based on the value of a flag. To avoid breaking everyone, we add
a (default) proto_toolchain target to @bazel_tools//tools/proto:toolchain that
is registered in a workspace suffix and add a migration flag to disable them (thus
requiring users to upgrade to a compatible @rules_proto version. At this point,
it is unclear whether we want to do this migration now (i.e. for Bazel 4.0) or
wait for a later release.

See bazelbuild/rules_proto#54

Updates #10005

[Yannic]

This is ready for review now.

[meisterT]

What's the state of this PR?

[philwo]

We're replacing the "WIP" label with the native GitHub "draft PRs". I thus converted this PR to a draft.

Please mark this as ready for review when it's ready, or close it if you no longer intend to work on it. Thanks! 👍

[Yannic]

Things have changed since opening this, so I'll close this and submit a new (updated) one.