Discovery of public key / certificates of Verifier by Wallet

For trust frameworks/jurisdictions in which Verifiers are out of scope for trust/compliance/certification, the Wallet needs a mechanism to discover the Verifier's public key / certificate when receiving a presentation request from the Verifier.

Possible option is a well-known document which can be retrieved using the FQDN of the Verifier's `redirect_uri`/`response_uri`.