feat: improve group filtering and only sync users who are members of the included groups

[StevanFreeborn]

closes #34
closes #26

• feat: accept group filter as odata $filter query string
• feat: use group filter to retrieve groups and only retrieve members of groups included by that filter
• fix: remove group filter
• tests: fix tests after group filter refactor
• docs: update README.md to reflect group filter and user sync behavior
• tests: write tests for syncing users via group membership
• docs: update example config with group filter

[codecov]

Codecov Report

Attention: Patch coverage is 94.85714% with 9 lines in your changes missing coverage. Please review.

Project coverage is 98.75%. Comparing base (4d53326) to head (da5e0f9).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Models/Processor.cs	86.15%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   98.84%   98.75%   -0.10%     
==========================================
  Files          11       10       -1     
  Lines        1912     1926      +14     
  Branches      154      149       -5     
==========================================
+ Hits         1890     1902      +12     
- Misses         16       17       +1     
- Partials        6        7       +1     

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