This PR introduces a feature flag, bridge/impersonate-enable, to gate the new multi-group impersonation feature during its development.

The multi-group impersonation feature is large and will be split into multiple smaller PRs. This flag will keep the feature hidden from users until all frontend and backend components are complete and merged, ensuring we don't expose an incomplete UI.

The flag is disabled by default. A follow up task to remove the feature flag will be opened when the feature is fully tested and ready for GA.