Update Item Assets Extension

[duckontheweb]

Related Issue(s):

• Closes Testing and documentation for item-assets extension #325

Description:

Updates docstrings and adds tests for the Item Assets Extension.

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 #564 (bb5042c) into main (e45af8e) will increase coverage by 0.14%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
+ Coverage   94.54%   94.69%   +0.14%     
==========================================
  Files          71       72       +1     
  Lines       10707    10755      +48     
  Branches     1067     1068       +1     
==========================================
+ Hits        10123    10184      +61     
+ Misses        412      395      -17     
- Partials      172      176       +4     

Impacted Files	Coverage Δ
pystac/extensions/item_assets.py	84.09% <71.42%> (+17.42%)	⬆️
tests/extensions/test_item_assets.py	100.00% <100.00%> (ø)

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