How to categorize this issue?

/area open-source
/kind enhancement

What would you like to be added:
Currently, shoot control plane migrations cause temporary downtime for the shoot cluster because ETCD needs to be backed up and deleted before being restored in the new seed cluster. During this time, the API server, along with all other control plane components, is also taken down. Although the workload within the shoot cluster continues running, it cannot be reconciled, scaled, or updated, leading to downtime since the control plane is unavailable to users.

We would like to support live Control Plane Migration (CPM), allowing migrations to happen without causing downtime for the API server, thereby preventing downtime for the users. This ensures that the shoot cluster remains fully operational, with continuous availability of control-plane for the users.

We(@acumino, @shafeeqes and @ary1992) conducted a POC on this, and it is feasible to implement. More details can be found here.

Why is this needed:
Prevent downtime during control plane migration, ✨ enabling support for more use cases and scenarios, such as 'seed draining/shoot evictions' or streamlined seed cluster deletions.