fix: rewrite Host header in dex round tripper

[okhaliavka]

Fixes #3975

In Istio, HTTP traffic is routed to a cluster based on Host header. Dex reverse proxy does not rewrite Host header, so traffic does not get routed to argocd-dex-server cluster and no cluster-level configuration (e.g. mTLS) is applied. Because of this, request to dex-server fails in environments where strict mTLS is enabled or where outbound traffic policy is set to REGISTRY_ONLY.

PR #6183 didn't fix the issue because I missed that we also have to rewrite host header in DexRewriteURLRoundTripper and screwed up my testing somehow. This time I tested it more thoroughly and looks like this should finally fix the problem.

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[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (42bdb5a) 49.19% compared to head (a968673) 49.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13500   +/-   ##
=======================================
  Coverage   49.19%   49.20%           
=======================================
  Files         248      248           
  Lines       42908    42909    +1     
=======================================
+ Hits        21109    21113    +4     
+ Misses      19684    19682    -2     
+ Partials     2115     2114    -1     

Impacted Files	Coverage Δ
util/dex/dex.go	58.02% <100.00%> (+0.52%)	⬆️

... and 2 files with indirect coverage changes

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[alexmt]

Thank you!