Add pos atx to post commitment

[fasmat]

Motivation

Closes #3556

Changes

• a "commitment" ATX is chosen when PoST is initialised
  • this new ATX is referenced in the NiPoSTChallenge but only for the initial ATX of a node
  • incoming ATX are verified to have a commitment ATX when they are initial and don't have one if they are not
• Instead of only using the Nodes public key as commitment for post, the digest of the concatenation of public key and the ID of the chosen commitment ATX are used
• I did some cleanup of the tests in the activation package
  • handler_test.go: now uses t.Run to isolate various test cases from each other more clearly
  • imho there are still too many tests cases within single tests (especially for SyntacticallyValidate); if I find some time I'll break that up in multiple smaller tests
  • there is too much shared setup between tests in the activation package in general, which makes it hard to modify or add tests without breaking several existing tests; if I find some time I'll try to get rid of some shared setup code

Test Plan

• All existing tests still pass / are updated to incorporate changes
• New tests where added for new functions / methods and to test new behaviour

TODO

• [x] Explain motivation or link existing issue(s)
• [x] Test changes and document test plan
• [x] Update documentation as needed

DevOps Notes

• [x] This PR does not require configuration changes (e.g., environment variables, GitHub secrets, VM resources)
• [x] This PR does not affect public APIs
• [x] This PR does not rely on a new version of external services (PoET, elasticsearch, etc.)
• [x] This PR does not make changes to log messages (which monitoring infrastructure may rely on)