Support pre-authorized code grant

[babisRoutis]

Currently, the issuer supports only the authorization code grant, given that by design it has been implemented as a resource server that is protected by any of the shelf authorization server.

To support pre-authorized code grant, under the above constraints, the following would be needed:

• Protect the issuer with two, possibly different, authorizations servers. One for authorization code grant and another for pre-authorized grant
• Issuer will advertise both authorizations servers, via credential issuer metadata, authorizations_servers claim
• Credential offers, would advertise the use of the first for grant authorization code & the second for pre-authorize code grant
• Credential endpoint would support access_token issued by either authorization server

Implementation
There are two possible ways to implement the above.

1. Create a minimal authorization server, exposing token endpoint and (authorization server) metadata and possibly token introspection endpoint
2. Embed the above into the credential issuer