FR: Ability to create multiple tailnets on the same domain

### What are you trying to do?

I have a few different networks that under no circumstance do I want them to have an access to each other, and I want to be able to access those networks using Tailscale (one at a time).
Using ACLs to do so is not an option because ACLs might be error-prone and someone might connect those environment by a mistake one day. Separating to different accounts is much safer in that regard.

### How should we solve this?

The ability to create multiple networks / sub-accounts under one Tailscale domain/account.

### What is the impact of not solving this?

Currently I can not use Tailscale for that use-case although the product is great and has tons of features that I really need (for example the Kubernetes operator).

### Anything else?

_No response_