feat: add proto3_optional to proto_custom_library (#3222)
feat: add proto3_optional to proto_custom_library (#3222)

This adds the proto3_optional enablement flag to proto_custom_library
which in turn enables it in proto_library_with_info and micro-Bazel
GAPIC targets.
googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator,googleapis/gapic-generator