Support OAuth 2 based authentication

### New feature, improvement proposal

Next to HTTP Basic Auth and TLS Certificates there should be support for OAuth tokens being retrieved first and then being sent along via `Authorization: Bearer ...` as defined in https://www.rfc-editor.org/rfc/rfc6750.

This requires additional requests for retrieving the token via a non-interactive or interactive OAuth flow.

Start by reviewing the existing HTTP Basic Auth and TLS Certificate authentication paths, then read RFC 6750 and compare its bearer-token requirements with the resolver's current request flow. Done means supporting OAuth token retrieval through the stated non-interactive or interactive flow and sending the resulting token as an Authorization: Bearer header.

Written by the indexing model from the issue text.