Fix workspaces frontend auth feature flag

MR: Fix workspaces frontend auth feature flag (!152454 - merged)

Description

In ca40cadf file ee/app/controllers/remote_development/workspaces_controller.rb, the frontend remote_development_namespace_agent_authorization feature flag check incorrectly used the user as the actor actor instead of the root namespace.

We need to fix that to use group actor, not user.

Since we don't know the group until the user selects a project, this had to be done as a new API call from the frontend code.

See Fix workspaces frontend auth feature flag (!152454 - merged) for details.

Acceptance Criteria

• Feature flag works correctly on frontend