Support ed25519ph for hashedrekord for Go 1.20

[haydentherapper]

Description

ed25519 signatures are not supported with the hashedrekord type, though they are supported with rekord. The reason is that ed25519 computes the digest as part of its algorithm, so the original artifact is needed to verify a signature. We return an error when an ed25519 key is used with hashedrekord - https://github.com/sigstore/rekor/blob/main/pkg/types/hashedrekord/v0.0.1/entry.go#L163-L166

Go 1.20 now supports ed25519ph, so only the artifact hash is needed to verify the signature.