Add API for direct robot charging requests

[luca-della-vedova]

Feature request

Description

There is currently no API for system integrators to submit a robot charging request. This is especially problematic since direct task requests bypass the implicit charging task insertion logic, instead if the robot doesn't have enough battery to perform the direct task assignment RMF will print a warning but still send the robot.
For direct task requests, the system integrator is expected to keep track of whether a robot has enough battery and charge it if needed. However, since currently there is no API to do so, the only way to go at the moment is to set the finishing_request of the fleet adapter to charge then wait until the battery state of charge is high enough before submitting the task request.

Implementation considerations

A schema for the charging task should be added here and deserialization should be updated to parse it when adding standard tasks.