Improve stac api item search extensions

[Ariana-B]

Remedy some oversights from my previous work on stac api extensions:

• support fields, sortby, and filter as GET request query parameters
• remove conformance classes that weren't actually implemented (and don't need to be)
• make extension handling logic more in line with the extension specifications

This is a band-aid fix; the entire stac api implementation needs to be reworked.

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📚 Documentation preview 📚: https://datacube-explorer--570.org.readthedocs.build/en/570/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.35%. Comparing base (aa0d9f3) to head (4bebdc9).
Report is 75 commits behind head on develop.

❗ Current head 4bebdc9 differs from pull request most recent head 2eb1fb4

Please upload reports for the commit 2eb1fb4 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #570      +/-   ##
===========================================
+ Coverage    86.29%   86.35%   +0.06%     
===========================================
  Files           26       26              
  Lines         3451     3511      +60     
===========================================
+ Hits          2978     3032      +54     
- Misses         473      479       +6     

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[omad]

Merged in another PR