Add ROS2 logger

[tropappar]

Added logging to stdout using RCLCPP_DEBUG.
Pretty much a copy of the StdCoutLogger.

[b-adkins]

Suggested change

	* displays all the transitions on the console.
	* displays all behavior node status transitions
	* on the console.

[b-adkins]

Oh hey, I wrote copy pasted one of these. Except mine logged to INFO.

So, I would like to suggest adding a member variable, optionally set in the constructor, to set the log level for transitions.

[b-adkins]

I have two more thoughts about the design:

1. The ROS logger would also be a useful parameter (member variable, set in the constructor). This maximizes the flexibility for the user -- they can use the ROS node's logger or a different logger (e.g. from rclcpp::get_logger("BT Transitions")). I have found multiple loggers, even in a single node, to be extremely useful to filter log message and show the DEBUG level selectively.
2. If a constructor took a BT::TreeNode::Ptr instead of a tree object, then it would make the logger extremely flexible. This way, an API user could instantiate the logger for a subtree of interest, or (with log level as a parameter) log different subtrees at different log levels or under different loggers.