Add option to disable automatic redirect to SAML login in environment-variable mode

How to use GitHub

• Please use the 👍 reaction to show that you are interested into the same feature.
• Please don't comment if you have no relevant information to add. It's just extra noise for everyone subscribed to this issue.
• Subscribe to receive notifications on status change and new comments.

Feature request

Which Nextcloud Version are you currently using: 32.0.3

Is your feature request related to a problem? Please describe.
In environment-variable mode, requesting /index.php/login (so going to nextcloud root when not logged in) triggers an unconditional redirect to /apps/user_saml/saml/login (and then WAYF/IdP). This confuses users and prevents them from reading service information/terms before initiating federated login.

Describe the solution you'd like
Provide a setting to disable automatic redirect from /index.php/login to /apps/user_saml/saml/login and display /apps/user_saml/saml/selectUserBackEnd instead.

Additional context
For the moment I bypassed the problem by forcing $showLoginOptions = true; in Application.php

Start in Application.php, where the issue currently forces $showLoginOptions = true, and trace the environment-variable login flow from /index.php/login to the SAML routes. Define the setting's behavior so automatic redirection can be disabled and /apps/user_saml/saml/selectUserBackEnd is shown instead; verify both enabled and disabled cases.