feat(ocpp): add EncodingMethodEnumType enum for signed meter value encoding methods

feat(ocpp): add EncodingMethodEnumType enum for signed meter value encoding methods

- Add EncodingMethodEnumType enum with OCMF and EDL values based on
  OCA Application Note and OCPP 2.0.1/2.1 specs
- Use enum in SignedMeterData interface and wire types (with string
  union for extensibility)
- Replace hardcoded 'OCMF' string constant with enum value
- Update all tests to use EncodingMethodEnumType consistently