Search returning new args

[SpacemanPaul]

Reason for this pull request

Add custom_offsets and order_by arguments to search_returning().

(N.B. order_by is still unimplemented)

See EP-13.

N.B. originally includes the changes in #1553 - that PR has been dropped and this one updated to remove those changes.

Proposed changes

• Add both args to abstract base class and implementations

• Implement custom_offsets argument in all index drivers.

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.44444% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 85.62%. Comparing base (2ee5434) to head (55c5d89). Report is 5 commits behind head on develop-1.9.

Files	Patch %	Lines
datacube/index/postgres/_datasets.py	93.33%	2 Missing :warning:
datacube/index/memory/_datasets.py	93.75%	1 Missing :warning:
datacube/index/postgis/_datasets.py	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1557 +/- ## =============================================== - Coverage 85.65% 85.62% -0.03% =============================================== Files 140 140 Lines 15175 15299 +124 =============================================== + Hits 12998 13100 +102 - Misses 2177 2199 +22 ```

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