fix(pkce): 18045 PKCE respect base href and Reauth to PKCE on token expire

[austin5219]

Fixes #18045

This prepends the server base href to the PKCE login auth flow and paths rather than hardcoding pkce paths off of '/'
This also fixes on token expiry and re-auth that the user is redirected to PKCE if configured rather than always sending the user to the dex login path.

This fix should be cherry-picked to 2.12

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
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• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[austin5219]

Here are screenshots showing the original issue and the fixes this pr has. Certain sections have been redacted to not leak confidential or sensitive information.

Login screen to show base href is '/argocd'

This is a deployment of argocd v2.12.4 configured to use pkce oidc auth. This shows the error where the redirect uri sent to the oidc provider does not include the base href

This PR now respects the base href when generating the callback uri, shown below

On the pkce callback, the user is now directed to applications with respect to the base href

On ID Token expiry, the re-auth will now respect if PKCE has been configured and attempt a pkce login rather than sending the user to the dex endpoint

[austin5219]

@crenshaw-dev could I get a re-review with the evidence attached above?