Protocol Buffer support lacks configurability

[shs96c]

Description of the feature request:

protoc configuration in bazel assumes a single configuration is used throughout the build. While this may be true in a small repo, it's possible for there to be more than one class of usage within a repo, eg.when using ServiceTalk and regular protobufs.

Right now, there's a proto_lang_toolchain which allows a single configuration per language. This isn't sufficiently granular.

Feature requests: what underlying problem are you trying to solve with this feature?

I'd like to compile protobufs for java using more than one configuration of protoc

What's the output of bazel info release?

3.4.1

Have you found anything relevant by searching the web?

There are multiple rulesets out there that attempt to address this, such as pubref/rules_protobuf and stackb/rules_protobuf Neither of these is ideal, and it seems a pity that bazel's own baked in rules aren't sufficient.

[Yannic]

This will most-likely done as part of the protobuf-starlarkification (#10005).

[github-actions]

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[github-actions]

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