Update SAR Extension

[duckontheweb]

Related Issue(s):

• Update extension pydoc strings to match new extension architecture #326
• Implement collection summary functionality #327
• Ensure migration happens correctly with SAR extension #262

Description:

• Updates docstrings for SAR Extension implementation
• Adds Collection summaries for SAR Extension
• Adds migration for pre-0.9 SAR fields:
  • sar:type -> sar:product_type
  • sar:polarization -> sar:polarizations

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 #556 (78767ca) into main (e4059c8) will increase coverage by 0.08%.
The diff coverage is 99.23%.

@@            Coverage Diff             @@
##             main     #556      +/-   ##
==========================================
+ Coverage   94.35%   94.43%   +0.08%     
==========================================
  Files          71       71              
  Lines       10409    10611     +202     
  Branches     1089     1092       +3     
==========================================
+ Hits         9821    10021     +200     
- Misses        419      420       +1     
- Partials      169      170       +1     

Impacted Files	Coverage Δ
pystac/extensions/eo.py	94.41% <ø> (ø)
pystac/extensions/sar.py	98.60% <98.57%> (-0.37%)	⬇️
tests/extensions/test_sar.py	100.00% <100.00%> (ø)

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