Validate connecting two instances of one Zone over KDS

[jakubdyszkiewicz]

Description

Usually, it's at most one instance of Zone CP (leader) that connects to a Global CP.
With Postgres or Kube storage it's might be fine for a short period of time - we might see some resource conflicts of double creates/updates and XDS Admin operation can be broken.

However, with in-memory storage the state will be completely broken.

Introducing such validation is a bit risky. It would have to rely on ZoneInsights, what if we have subscription that is "stuck". It takes some time to clean them up. We could at least prevent this for in-memory storage