feat: Support PAT using client credentials

[joachimvh]

Updates the UMA server to use PAT as identification method for the RS.

The following process should be followed:

1. RO registers RS on AS. This is doing following the OIDC dynamic client registration format. The RO should identify themselves using the authorization header, the same as for the policy API (OIDC tokens and raw WebID are supported).
2. The AS returns the id/secret combination.
3. The RO sends these to the RS. The implementation here makes use of the CSS account system: specific account parameters need to be updated to trigger the changes.
4. The RS uses these credentials to request a PAT from the AS, using the token endpoint.
5. The returned token needs to be used when performing a protected operation (registering a resource/requesting a ticket/introspection).