Create a new branch for usage with BT V3 (Humble package)

[xkaraman]

This PR suggest to create a new branch (can only submit it to master though) to use this example with BT version 3, because as of today there is no BT4 package in ROS 2 Humble version (LTS). One must built BT4 from source.

The packages is called behaviortree_cpp_v3 instead of behaviortree_cpp in this version, and some code changes are required due to API compatibility.

Maybe an updated Readme that clarifies that the master branch requires BT4 is required to avoid any misunderstanding (see #2)