webserver: support authentication on the webserver

[hnousiainen]

About this change - What it does

Support requesting authentication on the web server. This is intended to limit exposure from local redirects, open proxies and/or call out functionality.

Why this way

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.36170% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.77%. Comparing base (5505b86) to head (8c1c2b0).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
pghoard/postgres_command.py	81.81%	2 Missing ⚠️
pghoard/webserver.py	90.90%	2 Missing ⚠️
pghoard/restore.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   91.01%   90.77%   -0.24%     
==========================================
  Files          31       31              
  Lines        4917     4999      +82     
==========================================
+ Hits         4475     4538      +63     
- Misses        442      461      +19     

Flag	Coverage Δ
	90.77% <89.36%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pghoard/config.py	92.26% <100.00%> (+0.09%)	⬆️
pghoard/object_store.py	98.76% <100.00%> (+0.04%)	⬆️
pghoard/restore.py	89.60% <85.71%> (-0.08%)	⬇️
pghoard/postgres_command.py	90.38% <81.81%> (-2.88%)	⬇️
pghoard/webserver.py	89.29% <90.90%> (-0.13%)	⬇️

... and 5 files with indirect coverage changes