Support grpc plugin in py_proto_library

[wiktortomczak]

Inspired by cc_proto_library(..., use_grpc_plugin=True).

Note: py_proto_library(..., use_grpc_plugin=True) depends on //external:grpc_python_plugin. The latter must be defined via bind() in WORKSPACE, eg.

bind(
name = "grpc_python_plugin",
actual = "@com_github_grpc_grpc//:grpc_python_plugin"
)

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

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[wiktortomczak]

Signed the CLA

[googlebot]

CLAs look good, thanks!

[xfxyjwf]

LGTM

Thanks for the patch!