feat: add validation of required fields in the bundle

[vishal-chdhry]

Summary

Ref #63

Sigstore bundle protobuf spec has field_behavior annotation which is used to describe field behavior, such as a field being required or immutable as described in AIP-203.

This PR adds verification of top-level required fields in the bundle.

TODO

Fix validation of oneof fields

Release Note

Add validation of required fields in the bundle

Documentation

[haydentherapper]

Overall LGTM, pending figuring out the issue with required one_of fields. Thanks for adding this!

[vishal-chdhry]

updated the PR @haydentherapper

[haydentherapper]

This is a good start, though the linked issue isn't completed by this. The library you were using originally is what we need to do verification of nested fields, otherwise we're just validating top level fields.

I'm fine with merging this as-is, I've changed the PR description to not close the issue.

[vishal-chdhry]

The library you were using originally is what we need to do verification of nested fields, otherwise we're just validating top level fields.

Would the better solution be to use the library after it has better support for oneof fields (The AIP does not state anything specific about how to handle oneof so I am not sure if we can get something added to the library)
or,
Should I just add validation for all the imported fields?

[haydentherapper]

I think the library would be the best path forward. Maybe we can post an issue there to see if they have suggestions for handling one-ofs in a way that is AIP compliant?

[vishal-chdhry]

logged an issue for this in the library: einride/aip-go#300

[haydentherapper]

@vishal-chdhry Do you want to rebase for the tests, and we'll merge this as is?