Add column for signature metadata

[romanstrobl]

PowerAuth server should support mutliple signature types, hence columns signature_data_method and signature_data_uri_id in table pa_signature_audit should be generified into a single JSON column e.g. signature_metadata.

Sample structure of contents of column signature_metadata after migration to JSON format:

{
    "signature_data_method": "POST",
    "signature_data_uri_id": "/authorize"
}

Other signature types will use dedicated JSONs. Let's prepare classes which model various signature types, for this release only the PowerAuth signature model class type.

[banterCZ]

A follow-up to #500