Initial hooks for additional STAC collection fields

[mpaget]

We'd like to enable explorer/stac as a provider endpoint for CEOS connected data assets. The CEOS folks have recommended the addition of the following fields into an explorer/stac Collection response:

• Providers
• Summaries
• Keywords

Ref: https://github.com/radiantearth/stac-spec/blob/master/collection-spec/collection-spec.md

This PR provides the initial hooks to support these three fields. Defaults (and hardcoded) to None values in this PR.

The content for each of these fields is not directly available in an ODC product def so will likely need to be compiled from ODC datasets, possibly in a similar manner to summary._stores._find_product_fixed_metadata() -- this can be a separate PR.

I don't have a full test/integration suite for explorer (yet) so I confess this PR is both a preliminary integration test and an opportunity for the package maintainers (and others) to help inform the approach. I look forward to your reviews and comments!

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📚 Documentation preview 📚: https://datacube-explorer--929.org.readthedocs.build/en/929/

[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.24%. Comparing base (16acb9d) to head (265b770).

Files with missing lines	Patch %	Lines
cubedash/_stac.py	82.60%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #929      +/-   ##
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+ Coverage    84.23%   84.24%   +0.01%     
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  Files           35       35              
  Lines         4199     4228      +29     
  Branches       526      533       +7     
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+ Hits          3537     3562      +25     
- Misses         471      473       +2     
- Partials       191      193       +2     

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