feat: can sign ocsp with ed25519

[Minipada]

Summary

This patch adds support for signing OCSP responses with Ed25519 keys in CFSSL's vendored golang.org/x/crypto/ocsp package.

What's Changed

• introduced OID for Ed25519 (1.3.101.112) in oidSignatureEd25519
• added support for ed25519.PublicKey in signingParamsForPublicKey
• updated signatureAlgorithmDetails to include x509.PureEd25519
• when ed25519.PublicKey is detected, the appropriate signature algorithm is selected and crypto.SHA512 is set as the hash (dummy value — EdDSA does not use prehashing)
• improved error message to reflect that EdDSA is now supported

Notes

• while Ed25519 doesn't require a prehash, crypto.SHA512 is currently used as a placeholder to satisfy interfaces — in practice, the signing code path avoids hashing when Hash(0) is detected.
• this is part of broader support for modern cryptographic algorithms within CFSSL’s OCSP response generation pipeline.

To use with golang/crypto#319