chore: Add a more complete basic example

[mulmarta]

The example has all kinds of PSKs, external commits and basic MLS rules.

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 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.77%. Comparing base (fde42ea) to head (4a285fc).

Files with missing lines	Patch %	Lines
mls-rs-provider-sqlite/src/psk.rs	0.00%	8 Missing ⚠️
...s-rs/src/storage_provider/in_memory/psk_storage.rs	0.00%	4 Missing ⚠️
mls-rs/src/group/proposal_cache.rs	0.00%	3 Missing ⚠️
mls-rs/src/psk.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
- Coverage   88.82%   88.77%   -0.06%     
==========================================
  Files         182      182              
  Lines       29174    29192      +18     
==========================================
  Hits        25914    25914              
- Misses       3260     3278      +18     

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