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

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.43%. Comparing base (a66559a) to head (2919f0e).
⚠️ Report is 575 commits behind head on main.

Files with missing lines	Patch %	Lines
pystac/asset.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #936      +/-   ##
==========================================
+ Coverage   94.42%   94.43%   +0.01%     
==========================================
  Files          83       83              
  Lines       12016    12056      +40     
  Branches     1136     1143       +7     
==========================================
+ Hits        11346    11385      +39     
  Misses        492      492              
- Partials      178      179       +1     

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