algorithm support tests

[jku]

sigstore-clients are not required to support specific signing algorithms (in the signing certificate).

• It would be good to still have tests for algorithm support specifically for verification (to be able gauge the level of support of a specific algorithm in the community)
• list of recommended algorithms: https://github.com/sigstore/architecture-docs/blob/main/algorithm-registry.md -- I'm mostly thinking of testing RSA, ECDSA and EdDSA in this issue
• These tests would have to be implemented in a way that clients can easily make it clear that lack of support for specific algorithms is intentional and not a bug: I'm not sure if the current xfail setup is easy enough (see tuf-conformance for a possible alternative)

See also #203