[DRAFT] Add RekorV2Client

[ramonpetgrave64]

Client support for Rekor V2: sigstore-python #289

Summary

Adds a new RekorV2Client class, for use as a library, not yet within the CLI.

With the same pending TODOs in #1387

• We are embedding the auto-generated types until we can find a home for them

Testing

• Unit tests that call live instances of RekorV2, only "alpha" for now
• Lints are not yet expected to pass, mainly due to type hints on the copy-pasted auto-generated types.

TODOS

• add more client methods: get_tile, get_entry_bundle, get_checkpoint.
• enable testing against more instances: staging, prod, local
• add extra assert in test after improve KindVersion compatibility #1370
• use these types in sigstore/protobuf-specs when ready (hopefully this week of May 19, 2025)
• remove workaround where the ...V_0_0_2 is changed to ...V002
• prefer sending the certificate, rather than the public key to Rekor

Release Note

• Added a RekorV2Client for posting new entries to a Rekor V2 instance.

Documentation

TODO