[Improvement] Add authentication with mTLS and/or OAuth 2.0

[tomwetjens]

Describe the improvement

Add section on optional authentication using mTLS and/or OAuth 2.0

Additional context

Reducing the attack surface by authenticating before XML is parsed, easier to prevent DDoS.

Expected behavior

Section added to the spec describing how we want to do mTLS or OAuth 2.0 if required by the implementors.

Additional context
This has been proposed by Dutch grid operators Alliander, Enexis and EDSN

[RobbenRiksen]

Temporary solution with mTLS has been implemented by GOPACS and Liander. Discussion in the TSC about a desirable solution for Shapeshifter is ongoing and will be addressed in the coming TSC meetings.