Trajectory- and waypoint-following agents

[jadecastro]

We currently have some support for following waypoints but no way of specifying trajectories and representations for agents in general (systems other than cars). This is a step toward making stochastic agents for verification/falsification. This is just to track some TODOs toward this.

1. Agent models

• For TrajectoryCar, create a struct (AgentData) containing details of the agent. As a minimal instantiation, we'd populate this with an AgentType enum (AgentType::{Car | Bicycle | etc..}) but this could contain meta-information like bounding boxes, parameters, etc. These could internally change the underlying behavior of FooAgent and what parameters it uses. Automotive TrajectoryAgents #8725

2. Support for trajectories

• Create an agent that follows a trajectory (e.g. Trajectory or PiecewisePolynomial), rather than a set of waypoints. This would be somewhat similar to what TrajectorySource does in addition to having the outputs of the existing TrajectoryCar. Trajectory specification for automotive-related TrajectoryAgents  #8597, Automotive TrajectoryAgents #8725

3. Waypoint following

• Extend support of TrajectoryAgent to encompass waypoints having speeds (others too?) and interpolating between waypoints via additional ctors in AgentTrajectory. Build AgentTrajectory from waypoints #8720
• Rename TrajectoryCar to PathFollowingAgent, adding the AgentData struct as a constructor argument. (keeping TrajectoryCar as-is)

@stonier

[jadecastro]

Closing, as all tasks for this issue are complete.