MpTokenMetadata should extend Metadata base class to enforce 1024-byte max length

[Patel-Raj11]

MpTokenMetadata currently extends Wrapper<String> directly and has no validation on the value it wraps:

  abstract static class _MpTokenMetadata extends Wrapper<String> implements Serializable {
      @Override
      public String toString() {
        return this.value();
      }
  }

Per the XRPL protocol, MPTokenMetadata is a hex-encoded blob with a maximum size of 1024 bytes (2048 hex characters). xrpld enforces this server-side and rejects transactions with temMALFORMED when the limit is exceeded.