Use extension-like API for CommonMetadata

[duckontheweb]

Related Issue(s):

• Move CommonMetadata to use Extensions API #481

Description:

• Moves CommonMetadata and Provider to their own modules
• Defines equality for Provider objects
• Changes CommonMetadata to use similar API to PropertyExtension
• Adds Asset.common_metadata property

TODO:

• Add tests to bring coverage back up

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #563 (5a72a2a) into main (3dff1b6) will decrease coverage by 0.02%.
The diff coverage is 97.20%.

@@            Coverage Diff             @@
##             main     #563      +/-   ##
==========================================
- Coverage   94.69%   94.66%   -0.03%     
==========================================
  Files          72       75       +3     
  Lines       10755    10722      -33     
  Branches     1068     1048      -20     
==========================================
- Hits        10184    10150      -34     
- Misses        395      396       +1     
  Partials      176      176              

Impacted Files	Coverage Δ
pystac/__init__.py	100.00% <ø> (ø)
pystac/asset.py	90.32% <71.42%> (+2.39%)	⬆️
pystac/provider.py	88.88% <88.88%> (ø)
pystac/common_metadata.py	92.72% <92.72%> (ø)
pystac/collection.py	93.47% <100.00%> (-0.01%)	⬇️
pystac/item.py	90.50% <100.00%> (-2.73%)	⬇️
tests/test_common_metadata.py	100.00% <100.00%> (ø)
tests/test_item.py	100.00% <100.00%> (ø)
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3dff1b6...5a72a2a. Read the comment docs.

[lossyrob]

A suggestion about TYPE_CHECKING import aliasing but otherwise LGTM