Refactor: remove betterproto dep

[woodruffw]

Our dependency on betterproto is a source of a few issues:

1. We currently depend on the 2.x series, which is only in beta (currently 2.0.0b6). This is both non-ideal and is incompatible with some Python installers, most notably uv: https://github.com/astral-sh/uv/blob/main/PIP_COMPATIBILITY.md#pre-release-compatibility
2. betterproto's handling of the Struct WKT is partially broken. I've tried to fix it, but round-tripping a JSON serialized message through a Struct is still impossible, which causes all kinds of problems for the (mostly in-toto) definitions that use Struct
3. Finally, the models and their APIs themselves aren't particularly Pythonic -- this is more of an issue with protobuf itself than betterproto, but results in development friction and impedance mismatches elsewhere that we have to paper over

As such, we should probably remove our dependency on betterproto in the medium term. There are a few blockers to doing this:

1. The Bundle, etc. models all come from protobuf-specs, which is currently built on top of betterproto for the Python bindings.
2. There are (probably) a few places we leak references to betterproto-generated models in Sigstore's public APIs

(1) is probably addressable either by re-modeling the relevant parts of protobuf-specs in pydantic, or by regenerating the Python bindings on top of the JSON Schema for protobuf-specs. (2) may require a major reversion.

CC @DarkaMaul since he noticed this 🙂

[woodruffw]

Not immediately useful, but saving this here so I don't forget it: https://github.com/criccomini/proto-schema-parser