config: use getters for authenticate, signout and forward auth urls

[calebdoxsey]

Summary

As part of removing side-effects from config option validation, this PR uses Get... methods for AuthenticateURL, SignOutURL and ForwardAuthURL.

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[codeclimate]

Code Climate has analyzed commit dfa01fb and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #2000 (dfa01fb) into master (1febaa8) will decrease coverage by 0.0%.
The diff coverage is 50.7%.

@@           Coverage Diff            @@
##           master   #2000     +/-   ##
========================================
- Coverage    59.7%   59.6%   -0.1%     
========================================
  Files         154     154             
  Lines       10565   10593     +28     
========================================
+ Hits         6310    6318      +8     
- Misses       3547    3559     +12     
- Partials      708     716      +8     

Impacted Files	Coverage Δ
authorize/authorize.go	64.7% <0.0%> (ø)
internal/autocert/manager.go	65.9% <0.0%> (-1.0%)	⬇️
internal/controlplane/xds_routes.go	73.9% <0.0%> (ø)
proxy/state.go	84.2% <33.3%> (-4.7%)	⬇️
proxy/handlers.go	91.1% <42.8%> (-2.0%)	⬇️
authenticate/state.go	77.6% <50.0%> (-2.1%)	⬇️
authorize/evaluator/evaluator.go	57.9% <50.0%> (-0.6%)	⬇️
proxy/proxy.go	74.4% <55.5%> (-6.5%)	⬇️
authenticate/handlers.go	83.9% <60.0%> (-0.5%)	⬇️
config/options.go	46.1% <60.0%> (-0.4%)	⬇️
... and 5 more