Add plugin functionality for readers, writers, and enhancements

[djhoese]

It was already possible to define a Satpy plugin package for composites (and therefore modifiers). This PR adds the ability to use that functionality for other components.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2138 (5720b0a) into main (15dbf3d) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2138      +/-   ##
==========================================
+ Coverage   93.88%   93.91%   +0.02%     
==========================================
  Files         283      283              
  Lines       43123    43258     +135     
==========================================
+ Hits        40488    40626     +138     
+ Misses       2635     2632       -3     

Flag	Coverage Δ
behaviourtests	4.78% <2.59%> (-0.01%)	⬇️
unittests	94.56% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
satpy/multiscene.py	91.90% <ø> (ø)
satpy/composites/config_loader.py	93.15% <100.00%> (+1.57%)	⬆️
satpy/readers/__init__.py	97.53% <100.00%> (+0.01%)	⬆️
satpy/tests/conftest.py	100.00% <100.00%> (ø)
satpy/tests/test_composites.py	100.00% <100.00%> (ø)
satpy/tests/test_config.py	97.08% <100.00%> (+2.46%)	⬆️
satpy/writers/__init__.py	88.59% <100.00%> (+0.10%)	⬆️

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[coveralls]

Coverage increased (+0.02%) to 94.508% when pulling 5720b0a on djhoese:doc-plugin-config into 15dbf3d on pytroll:main.

[djhoese]

See pytroll/satpy-composites-plugin-example#1 for the updated example repository.

[mraspaud]

Great work! A few questions and suggestions for improvement.

[mraspaud]

LGTM