Allow root admins to delegate login policy management to space admins

We would like Spacelift to support optional delegation of login policy management to space admins.

In larger organisations, login policy ownership often needs to sit closer to the teams who own and operate individual spaces. This is especially important where spaces are created dynamically through Terraform, or where child spaces are managed independently and root admins do not have enough context or visibility to manage every login policy centrally.

The current model requires root admins to manage login policies for all spaces. That creates an operational bottleneck, slows down onboarding, and makes it harder to scale Spacelift across multiple teams, business units, or platform domains.

Ideally, root admins should be able to decide whether login policy management can be delegated on a per-space basis. Delegated space admins should only be able to manage login policies within their own space, or within explicitly permitted child spaces, without affecting the root space or sibling spaces.

Root admins should retain central control, visibility, and the ability to revoke delegation if needed.