cmd/tailscale/cli,ipn,ipn/ipnlocal: add AutoExitNode preference for automatic exit node selection

With this change, policy enforcement and exit node resolution can happen in separate steps,
since enforcement no longer depends on resolving the suggested exit node. This keeps policy
enforcement synchronous (e.g., when switching profiles), while allowing exit node resolution
to be asynchronous on netmap updates, link changes, etc.

Additionally, the new preference will be used to let GUIs and CLIs switch back to "auto" mode
after a manual exit node override, which is necessary for tailscale/corp#29969.

Updates tailscale/corp#29969
Updates #16459

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/tailscale/cli/cli_test.go

@@ -971,6 +971,10 @@ func TestPrefFlagMapping(t *testing.T) {
 			// Used internally by LocalBackend as part of exit node usage toggling.
 			// No CLI flag for this.
 			continue
+		case "AutoExitNode":
+			// TODO(nickkhyl): should be handled by tailscale {set,up} --exit-node.
+			// See tailscale/tailscale#16459.
+			continue
 		}
 		t.Errorf("unexpected new ipn.Pref field %q is not handled by up.go (see addPrefFlagMapping and checkForAccidentalSettingReverts)", prefName)
 	}