[OID4VP] Verifier endpoint for PresentationSubmission

[yshyn-iohk]

Proposed feature

This issue is a subtask of the hyperledger/identus#16

Feature description

When using the direct_post response mode, the wallet sends an HTTP POST request to the verifier's endpoint.
The Cloud Agent must provide this endpoint to receive the VP.
Once the VP is received, the verifier should run validation to determine whether it is valid.

It can be helpful to have another endpoint from which the verifier could query the record to see the VP submission and status.