An EmbeddedAppConnector is added that when configured observes DNS
responses from the PeerAPI. If a response is found matching a configured
domain, routes are advertised when necessary.
The wiring from a configuration in the netmap capmap is not yet done, so
while the connector can be enabled, no domains can yet be added.
Updates tailscale/corp#15437
Signed-off-by: James Tucker <james@tailscale.com>
