BehaviorTree serialization and desiarilizaiton accros agent communications

The Behavior Trees produced by the InteractionSolver may contain _**custom**_ compute nodes that read and write to the BehaviorTree blackboard.
This makes such nodes hard to serialize as JSON (because custom code is not JSON representable).

We need a method to persist and load BehaviorTree objects that can be sent as byte messages across agent messages between InteractionSolver and UserAssistant.

In the simplest case this is solvable through `pickle`, but other solutions are welcome.