Feat(#1588): Processing extension

[guillemc23]

Related Issue(s):

• Processing extension #1588

Description:
Adds the Processing extension to the available pystac.extensions, adapted from https://github.com/stac-extensions/processing

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files)
• Tests pass (run pytest)
• 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]

Codecov Report

❌ Patch coverage is 0% with 199 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.18%. Comparing base (065a631) to head (59ebff6).

Files with missing lines	Patch %	Lines
pystac/extensions/processing.py	0.00%	199 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1589      +/-   ##
==========================================
- Coverage   92.32%   90.18%   -2.15%     
==========================================
  Files          55       56       +1     
  Lines        8381     8580     +199     
  Branches      966      970       +4     
==========================================
  Hits         7738     7738              
- Misses        457      656     +199     
  Partials      186      186              

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