feat(ocpp): add signing prerequisites validation and EC curve auto-derivation

feat(ocpp): add signing prerequisites validation and EC curve auto-derivation

- Add deriveSigningMethodFromPublicKeyHex: extracts EC curve OID from
  ASN.1 DER public key and maps to SigningMethodEnumType
- Add validateSigningPrerequisites: checks public key presence, curve
  detection, and config/key curve consistency
- Integrate validation in OCPP 2.0 (OCPPServiceUtils.buildMeterValue)
  and OCPP 1.6 (readSigningConfigForConnector) signing paths
- On validation failure: log warning and gracefully fallback to unsigned
  meter values instead of producing invalid signed data
- Add unit tests for deriveSigningMethodFromPublicKeyHex and
  validateSigningPrerequisites covering all paths