Plugin for attachable cart

[luca-della-vedova]

New feature implementation

Implemented feature

This PR adds a simulation plugin for simulating carts / trolleys that can be attached and towed around by a robot.

Implementation description

The plugin uses the dispenser / ingestor workflow but is slightly different from the normal dispenser usecase.
Specifically, a cart has to be able to be both "dispensed" (i.e. picked up from the robot) and "ingested" (dropped off). Hence it behaves as both a dispenser and ingestor.
The implementation itself uses DetachableJoint components to attach the robot to the cart when a dispenser request is received, to detach it when an ingestor request is received. It also published dispenser and ingestor states / responses to integrate with the rest of the RMF stack.

A few implementation caveats:

• The joint will be created as soon as the dispenser request is received, attaching the two items, hence care must be taken in the navgraph creation / attachable_cart model positioning to make sure the robot is in a feasible position. For example for an item that is towed, the model should be just behind the navgraph waypoint, for an item that the robot picks up from the bottom they should sit on top of each other.
• The item should be marked as "dispensable" as documented in Checking for models with dispensable in names to not be considered an obstacle by slotcar #74, to make sure slotcar's collision system doesn't consider it an obstacle

At the moment the plugin is only implemented for gz.

[luca-della-vedova]

Pending a refactor to use perform_action instead of dispensers and ingestors

[luca-della-vedova]

Alternative implementation in #120 was merged, closing