Add filter to Item and Collection get_assets method

[pjhartzell]

Related Issue(s):
#873

Description:
Adds ability to filter Item and Collection Assets by media_type and role via get_assets() method.

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

Base: 94.37% // Head: 94.38% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (297e415) compared to base (017ddfd).
Patch coverage: 95.45% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
+ Coverage   94.37%   94.38%   +0.01%     
==========================================
  Files          83       83              
  Lines       12015    12055      +40     
  Branches     1136     1143       +7     
==========================================
+ Hits        11339    11378      +39     
  Misses        496      496              
- Partials      180      181       +1     

Impacted Files	Coverage Δ
pystac/asset.py	84.21% <50.00%> (-1.91%)	⬇️
pystac/collection.py	93.43% <100.00%> (+0.55%)	⬆️
pystac/item.py	89.05% <100.00%> (+0.33%)	⬆️
tests/test_collection.py	99.62% <100.00%> (+0.01%)	⬆️
tests/test_item.py	99.56% <100.00%> (+0.02%)	⬆️

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