Support for AIP 203 annotations?

Hi there! Thanks for creating and maintaining this package -- I'm currently looking into integrating protobuf messages into one of the Python projects I work on, and this looks like a really nice interface.

As part of designing the messages I'm planning on using, I'd like to be able to use [AIP 203-style](https://google.aip.dev/203) annotations to signal that particular fields are required on the wire. I can see these annotations show up in other codegen backends (like Go and Java), but they don't seem to have any effect on betterproto's generation.

Full context: https://github.com/sigstore/protobuf-specs/pull/41

Is this something that's currently possible with `betterproto`? If not, is there interest in adding support for it? If so, it's something I could take a stab at 🙂

Thanks in advance!