appc,ipn/ipnlocal: install conn25 DNS routes when using exit node

We were early-returning when the node was using an exit node, before
Connectors 2025 split DNS routes were calculated and installed.

Now we assemble the routes first, then install them in both exit node
and non-exit-node contexts. The returned resolvers set UseWithExitNode
to true even though as of today, we believe they should be installed in
all cases without regard to that boolean value. With the boolean, we
preserve the flexibility to toggle behavior without touching ipnlocal.

We also add a TODO to turn the extra split DNS route gathering into a
feature hook (tailscale/corp#37125).

This does not affect appc connectors, which receive split DNS routes,
and the UseWithExitNode value directly from control.

Updates #16384

Signed-off-by: Michael Ben-Ami <mzb@tailscale.com>