We need to be able to provide the ability for the GUI clients to resolve and set
the exit node IP from an untrusted string, thus enabling the ability to specify
that information via enterprise policy.
This patch moves the relevant code out of the handler for `tailscale up`,
into a method on `Prefs` that may then be called by GUI clients.
We also update tests accordingly.
Updates https://github.com/tailscale/corp/issues/4239
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
f8a4df66de