Allow multiple proto toolchains within a single bazel project

[jklukas]

What language does this apply to?

Bazel

Describe the problem you are trying to solve.

I have a monorepo using Bazel where multiple different versions of protoc need to be in the mix. I have Java code that relies on protobuf-java == 3.25.5, so want to use protoc 25.5 for java proto libraries, while I have python code that relies on an even older version of protoc.

If I try to configure the proto toolchain with one of these earlier protoc versions, it breaks other modules. For example, the latest rules_scala picks up the same protoc and produces a ScalaDeps.java that is incompatible with its own needs.

Describe the solution you'd like

I want to be able to configure multiple proto toolchains within a single bazel monorepo and select a proto toolchain at the target level, so that I can opt in subsets of my monorepo to specific older protoc versions.

Describe alternatives you've considered

Additional context

[zhangskz]

This is something we might consider for prioritization, but don't have active plans to do this at the moment yet. Thanks for the feature request!