proto3 'optional' keyword support

Currently `optional Timestamp` and `optional Duration` generate non-nullable types.
However proto3 'optional' keyword means nullable types.

Could be nice to fix this issue to be able to have nullable and non-nullable proto3 types processed correctly.

https://github.com/protocolbuffers/protobuf/issues/2168
https://github.com/protocolbuffers/protobuf/issues/8822

Thanks in advance

No file or test is named. Start by tracing how proto3 optional fields become Dart types, then read the linked protobuf issues for the intended semantics. Done means optional Timestamp and Duration fields generate nullable types while non-optional fields remain non-nullable, with coverage for both cases.

Written by the indexing model from the issue text.