ipn/{ipnext,ipnlocal},feature/conn25: add ExtraDNSRoutes hook for conn25 split DNS

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dnsConfigForNetmap called appc.AppDNSRoutes directly to synthesize the
conn25 split DNS routes from the self node's capability map, which was
the one place node_backend.go depended on the appc package and was
marked with a TODO to make it a hook.

Add ipnext.Hooks.ExtraDNSRoutes, which returns extension-supplied split
DNS routes. authReconfigLocked calls it and passes the result into
dnsConfigForNetmap, which adds the routes alongside the netmap's own
routes with the same UseWithExitNode handling as before.

The hook takes no arguments: the conn25 extension computes the routes
in its OnSelfChange handler and clears them on a switch to a different
node, and it already tracks the AppConnector.Advertise pref from
ProfileStateChange. Both of those hooks fire before the reconfig that
consults ExtraDNSRoutes, so the state is current. The conditions are
unchanged: no routes when this node itself advertises as an app
connector, and none without the experimental capability.

Updates tailscale/corp#37125

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I6c2e9f4b8a1d3e7f5c0b2a9d8e6f4c3b1a7d5e9f

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