Add support for custom options on: oneofs, enums, and enum values

[jcready]

Fixes: https://github.com/timostamm/protobuf-ts/issues/37 and https://github.com/timostamm/protobuf-ts/issues/537

[fython]

May I know the latest status of this PR? I think these cool changes can bring benefits to many projects. I am trying to add description text by enum value options for some protobuf enum defined for objects status, and I read these options value in order to show on pages.