ipn/{ipnlocal,localapi}: add API to toggle use of exit node

This is primarily for GUIs, so they don't need to remember the most
recently used exit node themselves.

This adds some CLI commands, but they're disabled and behind the WIP
envknob, as we need to consider naming (on/off is ambiguous with
running an exit node, etc) as well as automatic exit node selection in
the future. For now the CLI commands are effectively developer debug
things to test the LocalAPI.

Updates tailscale/corp#18724

Change-Id: I9a32b00e3ffbf5b29bfdcad996a4296b5e37be7e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

client/tailscale/localclient.go

@@ -1418,6 +1418,15 @@ func (lc *LocalClient) CheckUpdate(ctx context.Context) (*tailcfg.ClientVersion,
 	return &cv, nil
 }
 
+// SetUseExitNode toggles the use of an exit node on or off.
+// To turn it on, there must have been a previously used exit node.
+// The most previously used one is reused.
+// This is a convenience method for GUIs. To select an actual one, update the prefs.
+func (lc *LocalClient) SetUseExitNode(ctx context.Context, on bool) error {
+	_, err := lc.send(ctx, "POST", "/localapi/v0/set-use-exit-node-enabled?enabled="+strconv.FormatBool(on), http.StatusOK, nil)
+	return err
+}
+
 // DriveSetServerAddr instructs Taildrive to use the server at addr to access
 // the filesystem. This is used on platforms like Windows and MacOS to let
 // Taildrive know to use the file server running in the GUI app.