SAML SSO authentication for merge request approval at instance level

See parent epic for more details SAML SSO authentication for merge request approval (&11084)

Proposal

SAML Auth is fundamentally different when enabled at Group level vs self-managed instance-level SAML. This epic is to enable SAML SSO authentication for merge request approval at instance level.

Instance level https://docs.gitlab.com/ee/integration/saml.html

Requirements

Currently settings for merge request approval auth is set at Group level https://docs.gitlab.com/ee/user/project/merge_requests/approvals/settings.html#require-user-password-to-approve. This will still be the case for instance level merge request approvals, allowing users to configure per group to enable this setting

Implementation plan