fix(ocpp): populate signingMethod field and add SigningMethodEnumType enum

fix(ocpp): populate signingMethod field and add SigningMethodEnumType enum

- Fix signingMethod being empty string in SignedMeterValueType payload
- Add SigningMethodEnumType enum per OCA Application Note Table 12 and
  OCPP 2.1 Appendix 7.4 with all 7 standardized values
- Propagate FiscalMetering.SigningMethod config variable to the signed
  meter data generator with fallback to ECDSA-secp256r1-SHA256
- Add OCPP 1.6 vendor key SigningMethod and read it in
  readSigningConfigForConnector
- Add MeterPublicKey and SigningMethod mappings to OCPP2_PARAMETER_KEY_MAP
- Use SigningMethodEnumType in all interfaces and type definitions
- Update README vendor-specific keys documentation