This adds support to make exit nodes and subnet routers work
when in scenarios where NAT is required.
It also updates the NATConfig to be generated from a `wgcfg.Config` as
that handles merging prefs with the netmap, so it has the required information
about whether an exit node is already configured and whether routes are accepted.
Updates tailscale/corp#8020
Signed-off-by: Maisem Ali <maisem@tailscale.com>
985535aebc