Support for OOB invitations with attachments

[etschelp]

aka connectionless credential offer, or connectionless presentation request.

Currently the following is supported:

Backend:

• indy v1 credential offer
• indy v2 credential offer
• dif credential offer
• indy v1 presentation request
• indy v2 presentation request
• dif presentation request

Frontend:

• indy v1 credential offer
• indy v2 credential offer
• dif credential offer
• indy v1 presentation request
• indy v2 presentation request
• dif presentation request

To support all types the following is needed:

1. A unified list of templates that can be used for issuing, containing references to ld-schemas and indy-creddefs
2. A new section in the frontend for presentation exchanges that behaves similar to the "credential issuance" section, because attached presentation requests do not belong to a connection yet when they are created.

Related rest endpoint is /api/proof-exchanges/proof-request/oob-attachment