Add SAML Auth for Approvals

What does this MR do and why?

Reintroduction of the SAML MR approval feature. This version fixes the bug with nested groups and adds a feature flag.

Add SAML Auth for Approvals

Add custom callback controller to handle IdP response

Refs:

• &11982 (closed) SAML SSO authentication for merge request appro... (&11982 - closed)
• &11084 SAML SSO authentication for merge request approval (&11084)
• #421959 (closed) [BE] Add optional SAML auth flow to MergeReques... (#421959 - closed)
• #421961 (closed) [FE] SAML based Merge Request Approval (#421961 - closed)
• !136772 (merged) Fix SAML approval group SSO path (!136772 - merged)
• !136838 (closed) Draft: Use root group's token for SSO redirect (!136838 - closed)

Changelog: added EE: true

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Before	After

How to set up and validate locally

Use: !130204 (merged)

MR acceptance checklist

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