FR: Allow disabling or customizing proto rule generation

Currently this always generates the following rules when a `proto_library` rule is present:

```
java_proto_library(
name = "my_java_proto",
deps = [":my_proto"],
)

java_library(
name = "my_java_library",
visibility = ["//:__subpackages__"],
exports = [":my_java_proto"],
)
```

It would be nice if this could be disabled by config or customizable so that it is compatible with other rules, e.g. https://rules-proto-grpc.com/en/latest/lang/java.html#java-proto-library

Start with the proto_library-triggered generation described in the issue and compare it with the referenced rules-proto-grpc Java integration. Trace how java_proto_library and java_library are produced, then define configuration or customization behavior that supports disabling or adapting those rules. Done means the generated rules can be controlled without breaking existing proto_library usage.

Written by the indexing model from the issue text.