Generalize test client

[nacx]

This refactors the Makefiles and extracts common environment variables to a file that can be independently imported.

It also refactors the OIDC interactions and encapsulates them in an OIDCTestClient to have cleaner tests and be able to reuse the logic in different e2e test suites. It also cleans up the logging and cookie tracking logic in independent RoundTrippers to be able to enable/disable the desired stuff as needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f5617dd) 89.34% compared to head (c9b5188) 89.37%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   89.34%   89.37%   +0.02%     
==========================================
  Files          17       17              
  Lines        1117     1120       +3     
==========================================
+ Hits          998     1001       +3     
  Misses         85       85              
  Partials       34       34              

Files	Coverage Δ
internal/authz/oidc.go	90.25% <100.00%> (+0.05%)	⬆️
internal/config.go	97.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[sergicastro]

Suggested change

	defaultTransport.TLSClientConfig = client.tlsConfig
	// setup the same TLS config into the transport used by the delegate client
	defaultTransport.TLSClientConfig = client.tlsConfig