Add Rekor Bundle message

[lkatalin]

Related to #261 Related to https://github.com/sigstore/rekor/issues/1943

I'm considering whether the RekorBundle struct would make more sense as bytes (like the canonicalized body), with the tradeoff of readability, so I may update this. (Or possibly we could have both, but that may be too verbose.)

Edit: Updated based on the conversation in #261 to be separate from the TLE message.