feat(flagd): migrate to new provider mode file and update e2e tests

[aepfli]

see open-feature/flagd#1504

this pr migrates the offline file provider in a backwards-compatible manner to its own provider.

Furthermore, we're updating the test harness to the new testbed approach.

[codecov]

Codecov Report

Attention: Patch coverage is 95.12195% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.91%. Comparing base (02dcfc0) to head (a5990a3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../flagd/resolvers/process/connector/file_watcher.py	94.28%	4 Missing ⚠️
...src/openfeature/contrib/provider/flagd/provider.py	80.00%	1 Missing ⚠️
.../contrib/provider/flagd/resolvers/process/flags.py	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   94.39%   93.91%   -0.48%     
==========================================
  Files          14       15       +1     
  Lines         749      789      +40     
==========================================
+ Hits          707      741      +34     
- Misses         42       48       +6     

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[aepfli]

@toddbaert @open-feature/sdk-python-maintainers please have a look