Support field extensions with use_allof_for_refs

[same-id]

References to other Issues or PRs

Have you read the Contributing Guidelines?

Yes

Brief description of what is fixed or changed

Allow extensions to be used with the recently added AllOf feature

Other comments

[johanbrandhorst]

Hi, thank you for this, I think it's probably a good time to add an example protobuf file that is generated with this option turned on, so we can see the difference introduced by this sort of chance. Please add a small new file to the https://github.com/grpc-ecosystem/grpc-gateway/tree/main/examples/internal/proto/examplepb folder. You'll need a new buf.gen.yaml file which sets this option, see the existing files in the directory for an example. Thanks!

[same-id]

Will do.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[same-id]

Hey, still haven't got to creating a whole new example protobuf.

I see that some other PRs extending my work have also made their way there:

#3240