Added support for motion strategies

[nkhosh]

In the Humanoid Interact task, this PR adds:

1. Motion strategy: a class which manages a sequence of contact keyframes, and transitions between them.
2. Success criteria (time limit, distance threshold and success sustain time) for each contact keyframe.
3. Ability to serialize/deserialize (save and load) motion strategies.
4. Sample motion strategies to be readily loaded into the task.