Add separate enterprise group owner feature flag to bypass placeholder confirmation

What does this MR do and why?

This MR breaks out the importer_user_mapping_allow_bypass_of_confirmation feature flag from !189546 (merged) so that admin placeholder confirmation bypass can be released in 18.1 and enterprise group owner bypass can be released later. It also makes it an ee flag because this feature is meant for enterprise group owners only.

How to set up and validate locally

Ensure specs in ee/spec/requests/groups_controller_spec.rb pass. No new features were added and the one place this flag is used has no notable effect on the app or API. Because the previous feature flag is enabled by default in the specs, the spec that ensures the group namespace setting is not exposed when the new flag is disabled demonstrates that it the param is properly controlled by the new flag.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.