Store Credential req - update credential field type

[rolsonquadras]

The credential field in store credential request is of string type. This adds encoding challenges when creating the request. Update this to use json.Rawmessage type.

https://github.com/trustbloc/edge-service/blob/843ad4fec21e0897c5ba5c4160326409321aeeed/pkg/restapi/vc/operation/models.go#L35