Support for mTLS

[ricklambrechts]

I have added support for mTLS client authentication [RFC8705].

With the new added getEndpoint function the correct (mtls) endpoint will be loaded from the provider config.
Added necessary variables for holding the mtls configuration, if wanted I can move it to a MtlsConfig class for example.

TODO:

• Use only authentication method
• Only for token, introspection, revocation and backchannel authentication endpoints.

List of common tasks a pull request require complete

• Changelog entry is added or the pull request don't alter library's functionality

[DeepDiver1975]

this is yet another rare usage case from my understanding. can we support such an implementation via a subclassing and overloading the required methods?

[flyingangel]

Also please add the capability to configure CURLOPT_SSLCERT and CURLOPT_SSLKEY or find a way to override Curl params in fetchURL()

[DeepDiver1975]

or find a way to override Curl params in fetchURL()

fetchURL() is protected - subclassing is the way to go