Make psycopg2 an optional dependency

[Ariana-B]

Reason for this pull request

Make datacube more lightweight to install by only requiring psycopg2 when accessing a pg database.

Proposed changes

• Move psycopg2 to [test] dependencies, and introduce new [postgres] option

• Switch from using psycopg2 Range types to the sqlalchemy version

• Closes #xxxx

• Tests added / passed

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

---

📚 Documentation preview 📚: https://datacube-core--1648.org.readthedocs.build/en/1648/

[SpacemanPaul]

Error message for no postgres driver should make it clear user can install psycopg2 OR psycopg2-binary.

Or the new psycopg package? I guess that probably works too?

[codecov]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 85.22%. Comparing base (db4b339) to head (d9d760b).
Report is 1 commits behind head on develop-1.9.

Files with missing lines	Patch %	Lines
datacube/drivers/postgis/_connections.py	50.00%	2 Missing ⚠️
datacube/drivers/postgres/_connections.py	50.00%	2 Missing ⚠️
datacube/drivers/postgis/_fields.py	50.00%	1 Missing ⚠️
datacube/drivers/postgres/_fields.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1648      +/-   ##
===============================================
- Coverage        85.24%   85.22%   -0.03%     
===============================================
  Files              144      144              
  Lines            15783    15787       +4     
===============================================
  Hits             13454    13454              
- Misses            2329     2333       +4     

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