Add support for ed25519 keys

[ramonpetgrave64]

Client support for Rekor V2: sigstore-python

Summary

Resolves #1376, #1378

Adds support for ed25519 keys. In the cryptography library, is not yet any support for ed25519ph operations.

• https://cryptography.io/en/44.0.3/hazmat/primitives/asymmetric/ed25519/#

Fixes the CI test for timestamp-authority to use the latest release, not the latest tag, since we could have new tags without associated release artifacts to download.

Release Note

• Added support for ed25519 keys.
• CI: Timestamp Authority tests use latest release, not latest tag, of
  sigstore/timestamp-authority

Documentation

None

[ramonpetgrave64]

@woodruffw @jku

[jku]

LGTM. Getting this tested should be easy... I don't think there is any problem just editing the trusted_root/trustedroot.v1.json asset and making sure one of the keys is ed25519 (you could add a new log instance for example) -- the test only tests loading the key but it's better than nothing