Support xcube multi-level datasets

[forman]

• New xcube multi-level dataset rules:
  • ml-dataset-meta: verifies that a meta info file exists and is consistent
  • ml-dataset-xy: verifies that the levels have expected spatial resolutions
  • ml-dataset-time: verifies that the levels have expected time dimension, if any
• Now supporting xcube multi-level datasets *.levels:
  • Added xcube plugin processor "xcube/multi-level-dataset" that is used
    inside the predefined xcube configurations "all" and "recommended".
• Directories that are recognized by file patterns associated with a non-empty
  configuration object are no longer recursively
  traversed.
• Introduced method Plugin.define_config which defines a named plugin
  configuration. It takes a name and a configuration object or list of
  configuration objects.
• Changed the way how configuration is defined and exported from
  Python configuration files:
  • Renamed function that exports configuration from export_configs
    into export_config.
  • The returned value should be a list of values that can be
    converted into configuration objects: mixed Config instances,
    dictionary, or a name that refers to a named configuration of a plugin.
• Node path names now contain the dataset index if a file path
  has been opened by a processor produced multiple
  datasets to validate.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 98.32215% with 5 lines in your changes missing coverage. Please review.

Project coverage is 97.46%. Comparing base (9c53a01) to head (0955eda).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/cli/engine.py	92.59%	0 Missing and 2 partials ⚠️
xrlint/plugins/xcube/util.py	95.55%	0 Missing and 2 partials ⚠️
xrlint/plugins/xcube/rules/ml_dataset_time.py	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   97.31%   97.46%   +0.14%     
==========================================
  Files          57       61       +4     
  Lines        2236     2441     +205     
  Branches      306      343      +37     
==========================================
+ Hits         2176     2379     +203     
+ Misses         32       31       -1     
- Partials       28       31       +3     

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