The new version of app connector (conn25) needs to read DNS responses
for domains it is interested in and store and swap out IP addresses.
Add a hook to dns manager to enable this.
Give the conn25 updated netmaps so that it knows when to assign
connecting addresses and from what pool.
Assign an address when we see a DNS response for a domain we are
interested in, but don't do anything with the address yet.
Updates tailscale/corp#34252
Signed-off-by: Fran Bull <fran@tailscale.com>
3b737edbf1