Support multiple plugins with the Bazel rules

[shs96c]

What language does this apply to?
Bazel rules within this repo

Describe the problem you are trying to solve.
The command line protoc allows for more than one plugin to be called at a time. The protoc toolchain in the bazel rules supplied by this repo only allows for a single plugin to be used. It would be beneficial to be able to use multiple plugins

Describe the solution you'd like
proto_lang_toolchain could be extended to support with a plugins attribute, allowing users to specify multiple protoc plugins.

Describe alternatives you've considered
Rulesets such as rules_proto_grpc already support using multiple protoc plugins, but those don't integrate into the official java_proto_library targets.

[jguamie]

What is your use case here? We are trying to understand why you need multiple protoc plugins.