[DOP-9720] - add FileListHWM, FileHWM

[maxim-lixakov]

Change Summary

• Add classes: FileListHWM, FileHWM in etl_entitites.hwm
• Add tests
• Update documentation

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

Merging #30 (964c6ee) into develop (baafda7) will decrease coverage by 0.08%.
The diff coverage is 95.60%.

@@             Coverage Diff             @@
##           develop      #30      +/-   ##
===========================================
- Coverage    96.19%   96.11%   -0.08%     
===========================================
  Files           38       40       +2     
  Lines          735      823      +88     
  Branches       122      137      +15     
===========================================
+ Hits           707      791      +84     
- Misses          14       16       +2     
- Partials        14       16       +2     

Files	Coverage Δ
etl_entities/entity.py	96.42% <100.00%> (ø)
etl_entities/hwm/__init__.py	100.00% <100.00%> (ø)
etl_entities/hwm/hwm.py	90.00% <100.00%> (ø)
etl_entities/old_hwm/file_hwm.py	89.65% <ø> (ø)
etl_entities/hwm/file/file_hwm.py	92.30% <92.30%> (ø)
etl_entities/hwm/file/file_list_hwm.py	96.66% <96.66%> (ø)

[dolfinus]

Добавь тесты на сериализацию/десериализацию.