Add proxy fix header handling for X-Forwarded-For etc.

[pjonsson]

This is #1085 cherry-picked to the main branch. The commit that fixed type annotations is omitted since there is no type signature on that function in 1.8.

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📚 Documentation preview 📚: https://datacube-ows--1090.org.readthedocs.build/en/1090/

[codecov]

Codecov Report

Attention: Patch coverage is 55.55556% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.58%. Comparing base (82c9c77) to head (100d230).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
datacube_ows/startup_utils.py	42.85%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1090      +/-   ##
==========================================
- Coverage   93.64%   93.58%   -0.06%     
==========================================
  Files          43       43              
  Lines        6510     6518       +8     
==========================================
+ Hits         6096     6100       +4     
- Misses        414      418       +4     

Files with missing lines	Coverage Δ
datacube_ows/ogc.py	85.29% <100.00%> (+0.10%)	⬆️
datacube_ows/startup_utils.py	90.32% <42.85%> (-2.25%)	⬇️

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[pjonsson]

@alexgleith seems my nginx rewrites URLs in the response for GetCapabilities already, so I can't test this. The git conflicts were just about functions being in different modules though.

[alexgleith]

Looks good to me.