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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

:exclamation: 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

```diff @@ 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