[FE] SAML based Merge Request Approval

Refer to [UX] Design for SAML based Merge Request Approval (#421960 - closed) • Unassigned for design decisions.

Implementation Plan

• Update approval modal to account for SAML SSO auth: ee/app/assets/javascripts/vue_merge_request_widget/components/approvals/approvals_auth.vue
  • Remove ability to use password(PW) auth when SAML is enabled (don't render PW auth UI)
  • Show UI to let user select their group's SAML Identity Provider to approve the merge request The group admin dictates the given SAML provider.