More flexible task definitions system using Phases as building blocks

[aaronchongth]

Feature request

Description

This would be a meta-issue tracking the effort and prototyping the dashboard to work with open-rmf/rmf_ros2#107, where we allow users to create their own task definitions using phases that have been predefined for their own fleet of robots and infrastructure.

• Figma prototype

[aaronchongth]

Per discussion, the initial Figma prototype should involve a user interface where

• phases from different fleets are provided, and can be distinguished from one another (eg. color, namespaces or different columns/buckets of phases)
• users can start crafting a task by clicking any of the listed phases, and the interface should display how the task that is under-construction looks like
• certain phases require other phases to precede it, eg. a Fleet A:Dropoff must require a Fleet A:Pickup before it in the chain of phases. However for the initial prototype's interactions, we will start with phases from fleet B and C being greyed out and not selectable, whenever the task starts with a phase from fleet A, and vice versa.