Fall back on item search in CollectionClient

[gadomski]

Related Issue(s):

• Closes Collections get_item could also search if supported #236

Description:

Fall back on item search if features isn't implemented for a collection client.

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[codecov-commenter]

Codecov Report

Base: 85.05% // Head: 85.54% // Increases project coverage by +0.48% 🎉

Coverage data is based on head (1825895) compared to base (00e4c98).
Patch coverage: 90.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #379      +/-   ##
==========================================
+ Coverage   85.05%   85.54%   +0.48%     
==========================================
  Files          11       11              
  Lines         823      830       +7     
==========================================
+ Hits          700      710      +10     
+ Misses        123      120       -3     

Impacted Files	Coverage Δ
pystac_client/collection_client.py	82.35% <90.00%> (+0.38%)	⬆️
pystac_client/stac_api_io.py	90.35% <0.00%> (+3.50%)	⬆️

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[pjhartzell]

Note: the get_item method description needs to be updated to reflect the additional logic