Generate unmarshallers always if a submessage has a fieldmask

[ysmilda]

With the migration to proto v2 from gogoproto we're running into issues with how we historically rendered field masks. We used the {"field_mask":["field1", "field2"]} while they are meant to be just {"field_mask":"field1,field2"}. To not break anything when migrating we need to make sure that both formats are accepted by the unmarshallers.

For marshallers/unmarshallers currently generated by protoc-gen-go-json this is already the case, but for the other messages that use the standard marshallers this doesn't happen.

The proposal is to have protoc-gen-go-json generate all marshallers/unmarshallers for messages with fieldmasks.