add support for proto3 optional fields

[wespen]

protoc v3.12.0 added support for the optional annotation to proto3 (see RC: https://github.com/protocolbuffers/protobuf/releases/tag/v3.12.0-rc1)

Trying optional fields generation now errors out with:
proto3 file that contains optional fields, but code generator protoc-gen-rbi hasn't been updated to support optional fields in proto3. Please ask the owner of this code generator to support proto3 optional.--rbi_out

[mdehoog]

Thanks for raising the issue. Waiting for lyft/protoc-gen-star/pull/85 to be merged to fix this.