bazel: py_gapic_library rule should support specifying grpc_service_config

[noahdietz]

The generator can consume the retry/timeout config from the grpc_service_config file by passing the file through a protoc option. This should be reflected on the surface of the py_gapic_library rule as a grpc_service_config attribute. This is necessary for migration of generation targets to the microgenerator as a majority of APIs have already been moved to this config.

Examples of other languages that have implemented this attribute in their rules include:

• https://github.com/googleapis/gapic-generator-go/blob/master/rules_go_gapic/go_gapic.bzl#L88-L91
• https://github.com/googleapis/gapic-generator-typescript/blob/master/rules_typescript_gapic/typescript_gapic.bzl#L41-L42

[busunkim96]

Closed by #526. Microgenerated Python libraries are now using the service config via Bazel.