Bazel: support @generated=omit #11226

Is your feature request related to a problem?

There is currently no simple way to use the (wonderful) new 1.64.0 feature @generated=omit from Bazel. Which leaves Bazel users with a superfluous dependency for a single annotation.

It might be possible to do so with a new toolchain, but that's a lot of effort for a minor feature. If that doesn't work either, it's probably impossible. This might also be related to #11097

Describe the solution you'd like

Either set it to omit by default or add an option to change it.

https://github.com/grpc/grpc-java/blob/v1.64.0/java_grpc_library.bzl#L95 looks like the right place; args.add("--rpc-plugin_opt=@generated={0}".format(<???>)) behind a conditional (only if non-default) should do the trick - with a new attribute in the rules below.

The ..._opt flag to set the CodeGeneratorRequest.parameter for a plugin is not well documented, but is referenced e.g. here: protocolbuffers/protobuf#4759 (comment) (EDIT: a better reference would be https://github.com/protocolbuffers/protobuf/blob/v26.1/src/google/protobuf/compiler/command_line_interface.h#L107-L175)

It can occur more than once. It might even be worth it to also use this instead of the lite/non-lite toolchains.

Describe alternatives you've considered

I tried a .bazelrc entry with build --protocopt=--rpc-plugin_opt=@generated=omit - but that failed. The descriptors are created before the plugin is called, the option is unknown at that point and the command fails.

Additional context