Add request_type identifier and configurable retreat_to_charger

[xiyuoh]

This PR

• Adds retreat_to_charger to the task planner params, enabling it to be configured from the fleet config
• Adds a virtual get function in RequestFactory and implement them to allow request factory identifiers to be retrievable. The identifiers we currently have are Charge and ParkRobot.

[Yadunund]

Maybe I'm missing something but the "automatic retreat to the charger" behavior in the fleet adapters has nothing to do with the task planning in rmf_task. The issue should be resolved by creating/resetting this timer

[xiyuoh]

@Yadunund You're right, the timer is where it happens. This PR primarily stores retreat_to_charger as a boolean in Constraints, along with the other battery-related constraints. A check is implemented in my other PR in the fleet adapter - if retreat_to_charger is disabled, then exit the timer callback. Should i be storing this param elsewhere?