fix(ocpp): conform signingMethod to OCA spec and fix template curve mismatch

fix(ocpp): conform signingMethod to OCA spec and fix template curve mismatch

- Set signingMethod to empty string when using OCMF encoding per OCA
  Application Note Table 11: SA field already in signedMeterData
- Add FiscalMetering.SigningMethod=ECDSA-secp256k1-SHA256 to
  keba-ocpp2-signed template to match the secp256k1 public key from
  OCA spec §5.3 example
- Update tests to expect empty signingMethod for OCMF payloads
- Remove unused SigningMethodEnumType imports from test files