Use exit node

[JS-hacs]

Please add the ability to use an exit node (not the already implemented advertise as exit node)

[lmagyar]

TS tests for running in a HA add-on, and disables this option: https://github.com/tailscale/tailscale/blame/46505ca338797ed4a4b902afd1f989c783000475/envknob/featureknob/featureknob.go#L62-L64

[lmagyar]

After the general ban on exit node and ssh in case of HA is removed from TS (tailscale/tailscale#16263), we can make it configurable.

I've made a PR, it is draft, because the changes are not relesed by TS yet. #517