Implement support for authentication via OAuth2 using the client
credentials "Client Credentials Grant" flow (4.4 of RFC 6749).
For this a new configuration section is added to the config file,
where the client_id, client_secret and token_url have to be
specified.
The impelmention does currently not support "refresh tokens", but
does support refreshing the token if an `expires_in` is present
in the token itself.
Corresponding unit tests have been added.
[1] https://datatracker.ietf.org/doc/html/rfc6749#section-4.4
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