Fleets

[xtfer]

This adds new commands for managing a Fleet of projects, which can then be piped into the multi command.

A Fleet is currently a named key in a file .platform/fleets.yaml, which contains a project key containing an array of project IDs. This can be edited manually or by using the following CLI commands:

• fleet:list: List all fleets and their projects
• fleet:add: Add a new fleet
• fleet:remove: Remove a fleet
• fleet:project-add: Add a project ID to a fleet
• fleet:project-remove: Remove a project ID from a fleet

This PR combines work @Kazanir did on the multi command with a Fleet service class and commands.