Auth: add Authorization Server URL in Service Access Token flow (v2)

[reinkrul]

Verifier DID is now used to derive the OAuth2 Authorization Server URL (convert did:web DID to URL, then use well-knowns), but this is not DID-method agnostic. We need to provide the authz server URL for DID methods which can't be 1:1 mapped to a domain/URL.

Impacts the integration with the EHR/application: it needs to look up the URL in the DID document and provide it to the API call.

[reinkrul]

Having the DID in the Nuts node's Authorization Server URL and requiring parties to register this URL in their DID documents causes awkward management: to register the auth server URL in a DID document service, they need to construct the URL.

Which, currently, means translating the did:web DID to an URL, to the following form:

https://domain.com/iam/{id}

This is also a did:web-specific form, we should probably change this to /{did} to include the full DID.

[reinkrul]

Replaced verifier DID withauthorization_server (URL)