[Idea] Make query filter optional when returning credentials

[kirahsapong]

Is your feature request related to a problem? Please describe.
When requesting credentials, a query param of issuer, subject or schema is required and the response is filtered accordingly.

Describe the solution you'd like
Would be great to make this query param optional, and return all credentials in the absence of a filter.

Describe alternatives you've considered
Currently traversing all issuer DIDs stored in the db and requesting credentials by issuer in order to get all credentials stored in the db

[decentralgabe]

fix is in #489

[nitro-neal]

#489 is merged in