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.

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

[codecov]

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 ⚠️
datacube/index/memory/_datasets.py	93.75%	1 Missing ⚠️
datacube/index/postgis/_datasets.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@               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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