Adds policy keys ExitNodeID and ExitNodeIP.
Uses the policy keys to determine the exit node in preferences.
Fixestailscale/corp#15683
Signed-off-by: Claire Wang <claire@tailscale.com>
Add a more generalized package for getting policies.
Updates tailcale/corp#10967
Signed-off-by: Claire Wang <claire@tailscale.com>
Co-authored-by: Adrian Dewhurst <adrian@tailscale.com>