CBOM: add custom fingerprints

[bhess]

Extend the fingerprint definition in cyclonedx-cryptography-2.0.schema.json to support custom fingerprint algorithms alongside standard hash algorithms.

Changes

• Replace the flat $ref: hash on certificateProperties.fingerprint and relatedCryptoMaterialProperties.fingerprint with a single central $defs/fingerprint definition
• $defs/fingerprint uses oneOf with two branches:
  • Standard Hash — alg + content (refs to existing hashAlgorithm / hashValue); fully backward compatible
  • Custom Fingerprint — customAlg + customContent for non-standard algorithms

Backward Compatibility

Existing documents with {"alg": "SHA-256", "content": "..."} satisfy the Standard Hash branch unchanged.