FR: Add a VPN on demand temporary disable toggle

### What are you trying to do?

VPN on demand works as expected, based on the parameters defined. But sometimes it might be necessary to temporarily disable it, even when the conditions are met. Currently it is "sticky", it will get enabled if the conditions are met.

### How should we solve this?

Adding a temporary disable feature would allow people to turn it off for a short period of time if necessary. Once the timer expires, the VPN on demand functionality can turn itself back on again.

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

Technically none. Although having this functionality can aid users in the scenarios where they intentionally want to not use VPN even when the on demand conditions are met.

### Anything else?

There's a small UX issue with how the VPN on demand functionality works, if the parameters are met. I've documented that more in a separate issue linked [here](https://github.com/tailscale/tailscale/issues/18059).