* Per review feedback, rename the nav2_controller to be nav2_lifecycle_manager

To avoid confusion with low-level controllers, be more explicit in the name.

* Avoid colliding node names; make navigation dialog's node name generic

* Add the lifecycle controller

* Generalize the nav2_controller code.

The only nav2-specifics are now the list of node names.

* Some reformatting

* Rename the Srv service to be more explicit; Add description to the package XML file

* Address more review feedback