[refactor] Modular tasks

Currently, rmf_task::Request is used to define a bunch of requests (loop, delivery, clean, chargebattery) that are later converted into rmf_fleet_adapter::Task within the fleet adapter. Each rmf_fleet_adapter::Task is composed of a string of phases present in src/rmf_fleet_adapter/phases. The definitions of these requests/tasks are rigid and the process of introducing new tasks is fairly involved.

The proposal is to move some of the Phases into the public API in rmf_task. These would serve as building blocks to describe tasks. Hence requests would be described as a collection of these phases. Each phase/block would also inherit from an error handling class that dictates how errors during the phase should be resolved. Similarly for when the phase is cancelled. Each phase would also have invariant_duration and estimate_finish methods to allow the task planner to estimate the duration of the phase and the state of the agent at the end of it.

Start by tracing how rmf_task::Request is converted into rmf_fleet_adapter::Task, then read the existing phase definitions under src/rmf_fleet_adapter/phases. Determine which phases can become public rmf_task building blocks and how error handling, cancellation, invariant_duration, and estimate_finish should be represented. Done means the task model supports composing requests from these phases without the current rigid conversion process.