Fix spatial extents

[SpacemanPaul]

Reason for this pull request

In the postgis driver, the dc.index.products.spatial_extent() method was returning None if there was no dedicated spatial index defined for the requested CRS.

This issue was identified during the OWS anti-meridian work.

Proposed changes

• Calculate extent in epsg:4326 (which is always available) if no dedicated spatial index defined for the requested CRS.

  • [x] Tests added / passed
  • [x] Fully documented, including docs/about/whats_new.rst for all changes

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📚 Documentation preview 📚: https://datacube-core--1662.org.readthedocs.build/en/1662/

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.22%. Comparing base (667f47f) to head (b546d80). Report is 1 commits behind head on develop-1.9.

Files with missing lines	Patch %	Lines
datacube/drivers/postgis/_api.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1662 +/- ## ============================================ Coverage 85.22% 85.22% ============================================ Files 144 144 Lines 15787 15787 ============================================ Hits 13454 13454 Misses 2333 2333 ```

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