[BE] Add setting for SAML based MR approval

Settings for SAML MR Approval

Add setting to toggle behavoir of SAML approval of merge requets (ON/OFF)

Implementation plan

1. Add column to group_merge_request_approval_settings for does_approval_require_saml_auth -> [DB] Add group MR approval setting for SAML bas... (!129780 - merged) • SAM FIGUEROA • 16.4
2. Update validation on ee/app/models/group_merge_request_approval_setting.rb to only allow enabling feature if a SAML provider is set up correctly Code checks if SAML is enabled at time of render
3. Update API to expose setting through lib/api/settings.rb
4. Update filter_attributes_using_license (ee/lib/ee/api/settings.rb) for setting does_approval_require_saml_auth, to make sure it's only enabled where allowed
5. Update all relevant spec files