FR: Mullvad Multihop exit nodes

### What are you trying to do?

I am trialling Mullvad integration for Tailscale. Mullvad supports a feature known as multi hop, there is no current means of configuring a multi-hop exit node in Tailscale from what I can see with the current integration.

### How should we solve this?

Implement the ability to have multi-hop Mullvad exit nodes, in an ideal world, with a UI to configure exit and entry for the multihop.

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

This is a requirement for me, so this is likely unusable for me unless I can implement a cross platform (Windows, Linux, Android) workaround.

### Anything else?

This should probably be made clear in the documentation.