WebAPI: Add resolutions and spatialUnits to mldataset

[clarasb]

This PR:

• is needed for Add indicator of zoomLevel and datasetLevel to map xcube-viewer#544, to access information on the resolutions and spatialUnits of MultilevelDatasets from xcube Server in xcube Viewer.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[forman]

All fine, but please write a CHANGELOG entry.

Although the code is covered by at least one test, none of them really asserts the correctness of the new settings. Consider writing a new unit test or enhancing one.

[forman]

This is actually approved, but CI still fails. Please fix.

[clarasb]

The tests related this PR have been adjusted.

[konstntokas]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.52%. Comparing base (921e831) to head (73abdc4).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1177      +/-   ##
==========================================
+ Coverage   89.51%   89.52%   +0.01%     
==========================================
  Files         280      280              
  Lines       21659    21661       +2     
==========================================
+ Hits        19387    19391       +4     
+ Misses       2272     2270       -2     

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