Support for other Rekor types

[woodruffw]

sigstore-python currently supports Rekor's hashedrekord type, which embeds the content hash (rather than full content), content signature, and signing certificate (or cert chain, or public key) in the Rekor entry.

There other other Rekor entries we should probably also support for verification, or at least handle and fail intelligibly. These are covered in Rekor's docs: https://github.com/sigstore/rekor/blob/main/types.md

[woodruffw]

DSSE here: #628.

[woodruffw]

This is too broad, and there's been ~no interest in other types besides Rekor, so I'm closing this in favor of individual issues.