Supporting virtual plugins and ease deserialization

[forman]

• Supporting virtual plugins: plugins provided by Python
  dictionaries with rules defined by the RuleOp classes.
• Added more configuration examples in the examples folder.
• Introduced utilities ValueConstructible and
  derived MappingConstructible which greatly simplify
  flexible instantiation of configuration objects and their
  children from Python and JSON/YAML values.

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 91.29412% with 37 lines in your changes missing coverage. Please review.

Project coverage is 95.92%. Comparing base (64e58e1) to head (92d816a).
Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
xrlint/processor.py	72.97%	9 Missing and 1 partial ⚠️
xrlint/util/importutil.py	62.96%	4 Missing and 6 partials ⚠️
xrlint/util/codec.py	96.53%	3 Missing and 4 partials ⚠️
xrlint/rule.py	90.47%	4 Missing and 2 partials ⚠️
xrlint/config.py	95.83%	1 Missing and 1 partial ⚠️
xrlint/plugin.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   96.13%   95.92%   -0.22%     
==========================================
  Files          51       52       +1     
  Lines        1813     2060     +247     
  Branches      240      269      +29     
==========================================
+ Hits         1743     1976     +233     
- Misses         35       47      +12     
- Partials       35       37       +2     

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