[DOP-11304] Make directory field in FileHWM optional

[dolfinus]

Change Summary

1. Make FileHWM.directory field optional. Update related tests.
2. Changed type if items in FileListHWM.value from RelativePath to AbsolutePath, to avoid storing different path types with and without directory specified. Validation that paths in .value should be consistent with directory is still here, it is just disabled then directory=None. Method get_absolute_paths was dropped.
3. Add missing method OldFileListHWM.as_new_hwm() and add related tests.

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

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d2d720c) 95.60% compared to head (43c37c2) 95.40%.

Files	Patch %	Lines
etl_entities/hwm/file/file_list_hwm.py	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #60      +/-   ##
===========================================
- Coverage    95.60%   95.40%   -0.20%     
===========================================
  Files           48       48              
  Lines         1069     1067       -2     
  Branches       184      186       +2     
===========================================
- Hits          1022     1018       -4     
- Misses          27       28       +1     
- Partials        20       21       +1     

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