cryptographic agility updates

[loosebazooka]

bundle 4.x should fail on p384/sha256

see: sigstore/protobuf-specs#566

[woodruffw]

Thanks for filing this @loosebazooka!

To enumerate some cases we'll need:

1. A v3 or lower bundle that contains a PKIX_ECDSA_P384_SHA_256 or PKIX_ECDSA_P521_SHA_256 signature should be verified correctly.
2. A v4 or higher bundle that contains either of the above should be rejected.

[haydentherapper]

Another TODO is that this needs to be a part of the client spec for how clients should handle >=v3 bundles, falling back, and for >=v4, rejecting.