FR: New "Always On" option VPN on Demand for wifi/cellular for iOS Client

### What are you trying to do?

Manually toggle Tailscale ON/OFF while VPN On Demand is enabled and has "Only On" selected, with specified Wifi networks. Currently VPN on Demand turns on when I am on my work wifi automatically, which works great. However I find whenever I am at home and needing to turn on VPN it wont allow me to do so because its not in the list of SSIDs for "Only On". If I flick the switch on, it turns back off immediately.

### How should we solve this?

Create a new option in VPN On-Demand titled "Always On", which accepts SSIDs where VPN will automatically turn on; but with this option I am not restricted from turning on TS on other wifi networks so I'm still able to manually turn on TS adhoc.

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

Limits the usability of VPN-on-Demand on networks not explicitly specified, I have to always turn off VPN-on-Demand if I want to connect at home which coincidentally also wipes out the saved SSIDs so when I turn it back on again I have to manually enter the SSIDs again.

### Anything else?

N/A