Add support for "--experimental_allow_proto3_optional" protoc option

[sjoerdvisscher]

I'd like to enable the --experimental_allow_proto3_optional protoc option. Maybe this is already possible, but I was not able to figure out how.

[aaliddell]

This will be supported in 3.0.0 #106:

python_proto_compile(
    name = "person_python_proto",
    protos = ["@rules_proto_grpc//example/proto:person_proto"],
    extra_protoc_args = ["--experimental_allow_proto3_optional"]
)

[sjoerdvisscher]

Perfect, thanks!