Simplify/xr engine valid time as generic aux coord

[pawel-wolff]

Description

This PR replaces the specific management of the aux coord valid_time by the generic aux_coords mechanism implemented in PR #969. This change as the following advantages:

• Removes a complex logic of FieldListTensor.make_valid_datetime() from the outside xr_engine module src/earthkit/data/indexing/tensor.py
• An essential part of the above logic, i.e. the discovering which dataset dimension the aux coord valid_time should depend on, is moved to src/earthkit/data/xr_engine/dim.py:DimHandler.active_time_dim_names()
• The option add_valid_time_coord=True now works with allow_holes=True (previously, this combination was raising a "not implemented" exception)

Several new tests for the option add_valid_time_coord=True have been added.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.77%. Comparing base (df9a4ce) to head (45e1163).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #970      +/-   ##
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+ Coverage    81.68%   81.77%   +0.09%     
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  Files          226      227       +1     
  Lines        15000    15082      +82     
  Branches       746      746              
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+ Hits         12252    12334      +82     
  Misses        2523     2523              
  Partials       225      225              

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[sandorkertesz]

@pawel-wolff, this PR is ready to be merged. Please sync with the latest develop code to go ahead with it.

[pawel-wolff]

Done.