[stable/dex] Add support to always display login screen

[maxlegault]

What this PR does / why we need it:

The current indentation of the OAuth2 configuration block doesn't allow
for extra properties to be added. This allows to enable the new
configuration that will always display the login screen, even though
only one connector is defined.

The default value (disabled) is also added to the values file for
documentation.

See also: https://github.com/dexidp/dex/releases/tag/v2.18.0
See also: dexidp/dex#1505

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/chart])

[k8s-ci-robot]

Hi @bernyfox. Thanks for your PR.

I'm waiting for a helm member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[vi7]

/approved
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bernyfox, vi7

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[vi7]

/ok-to-test