Create experimental flag to include source info in proto descriptors

[Yannic]

protoc has an option to include source info (e.g. comments) in the
generated descriptor sets. Well done proto generators also relay the
comments into the generated source so IDEs that do source indexing
and pick them up to provide contextual tooltips/documentation/etc.

This change adds an experimental flag to include this info in the
descriptors generated by proto_library so we can collect some
data whether including source info by default is feasible.

See #9337 for more context.

//cc @thomasvl

[lberki]

After some more discussion with @oquenchil and @meisterT , we are kind of afraid that we'd never be able to flip this flag within Google.

There is a way around that, which is to make descriptor set generation use a regular proto_lang_toolchain through a dependency instead of the ad hoc one created here:

https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java#L359

and then we can use a different one within Google and with Bazel.

However, that's a lot of ceremony. So how about this course of action:

1. We get this change submitted
2. We run benchmarks within Google to figure out if flipping the flag is feasible or not
3. If yes, we flip the flag internally, then in Bazel
4. If not, we got with the above plan with proto_lang_toolchain rules

WDYT, @Yannic , @oquenchil , @meisterT ?

[meisterT]

Sounds like a simple plan, so I like it :-)

[Yannic]

The purpose of the flag is to see if including source info by default is feasible or not, so your plan SGTM.

[alexeagle]

@meisterT FYI I'm getting a request to add support for this experimental flag (aspect-build/rules_lint#314) which makes me wish for a tracking issue for removing the experimental_ prefix, following @lberki proposed plan above.