FR: Enable blacklisting of individual WiFi networks

### What are you trying to do?

As Tailscale's popularity increases, more and more businesses are starting to block Tailscale traffic on their networks. In the case of using your mobile device it makes it necessary to disable WiFi in order to push traffic out via mobile data instead.
In my case, my employer blocks Tailscale at all our corporate sites when accessed over the corporate WiFi. So every day I need to repeatedly disable WiFi when I need to access anything over Tailscale and then re-enable it when I need to access devices on the corporate network.

### How should we solve this?

Add the ability to the client to blacklist any given WiFi network based on SSID. When your device connect to a WiFi network the Tailscale client looks to see if it is in the blacklist. If it is, then it will route its traffic out the mobile data interface instead. This would allow Tailscale to continue to work but also allow non Tailscale traffic to still go out via the likely cheaper/faster WiFi network.
It may even be possible to automate the process, if it cannot establish a connection over WiFi then it defaults to mobile data.

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

Worsening pain for every user as more and more businesses and free wifi networks start blocking Tailscale due to their misconception that VPN = illegal behaviour.

### Anything else?

_No response_