Fix #971: Add column for signature metadata

- Add a test
- Introduce more generic approach

powerauth-java-server/src/test/java/io/getlime/security/powerauth/app/server/service/model/signature/SignatureMetadataConverterTest.java

@@ -30,7 +30,7 @@
  * Test class for SignatureMetadataConverter.
  * This class tests various aspects of converting SignatureMetadata to and from its serialized JSON form.
  *
- * @author Your Name
+ * @author Jan Dusil
  */
 public class SignatureMetadataConverterTest {