Use the OIDC Discovery end session endpoint if present

[nacx]

Currently, the end session URI present in the OIDC Discovery response was not used. This PR fixes this and uses the URI if none was configured.

[codecov-commenter]

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage        ?   90.33%           
=======================================
  Files           ?       24           
  Lines           ?     1573           
  Branches        ?        0           
=======================================
  Hits            ?     1421           
  Misses          ?      108           
  Partials        ?       44           

Files	Coverage Δ
internal/authz/oidc.go	93.61% <100.00%> (ø)

[sergicastro]

Nice, good catch

[istio-testing]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nacx, sergicastro

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