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

[alexgleith]

This is a low-risk and high-reward feature that adds another environment variable that turns on the "proxy_fix" middleware, which ensures the app understands what URLs it should return as part of the getCaps and other responses.

Tested in production for DE Pacific.

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📚 Documentation preview 📚: https://datacube-ows--1085.org.readthedocs.build/en/1085/

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.

Project coverage is 93.28%. Comparing base (5a0fbf4) to head (f298a2e).
Report is 10 commits behind head on develop-1.9.

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

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1085      +/-   ##
===============================================
- Coverage        93.33%   93.28%   -0.05%     
===============================================
  Files               53       53              
  Lines             7032     7040       +8     
===============================================
+ Hits              6563     6567       +4     
- Misses             469      473       +4     

Files with missing lines	Coverage Δ
datacube_ows/feature_info.py	90.54% <100.00%> (ø)
datacube_ows/ogc.py	86.13% <100.00%> (+0.10%)	⬆️
datacube_ows/startup_utils.py	90.32% <42.85%> (-2.25%)	⬇️

[pjonsson]

@alexgleith if I understand your description, GetCapabilities will return broken results without this. Does this already work for the main branch, or should this commit also be merged there?

[alexgleith]

Hey @pjonsson this was merged into the main branch too.

[pjonsson]

@alexgleith are you sure about that? If I git grep PROXY_FIX on latest master, I get no hits.

[alexgleith]

@alexgleith are you sure about that?

Nope, not sure! I think it must have been the other fix about cache-control headers that was merged into both.

Happy to review a PR that adds this to the 1.8 branch.