Enforce RFC rules on branch and reinit

[mulmarta]

Enforce that branch and reinit cannot add clients. Enforce that reinit cannot remove clients.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.62044% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.49%. Comparing base (efc2611) to head (461f5a6).

Files with missing lines	Patch %	Lines
mls-rs/src/group/resumption.rs	95.62%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
- Coverage   88.49%   88.49%   -0.01%     
==========================================
  Files         182      182              
  Lines       28969    28992      +23     
==========================================
+ Hits        25636    25656      +20     
- Misses       3333     3336       +3     

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