Extend filaments support to better support MMUs

[dc42]

Currently, filaments are associated with extruders even though M701/702 act on the current tool. This doesn't work well with MMUs that can feed different filaments through a common extruder. The current work around is to map multiple extruder numbers to the same driver; however this causes issue when stall detection is used to load filament because RRF wasn't designed to support this. See https://forum.duet3d.com/post/353387.

One way to improve on this would be:

1. Introduce the concept of feeders, where the combination of (extruder#, feeder#) defines a filament source and path. For now a feeder is just a number.
2. M563 would take an optional additional parameter (default 0) specifying the feeder number for the first extruder number mentioned (or possibly the feeder numbers of all extruders mentioned).
3. The M701/702/703 filament management mechanism would map each (extruder, feeder#) combination to a filament instead of mapping just an extruder number to a filament. The filaments.csv file would be changed to reflect this. For backwards compatibility, existing filament mapping files would be treated as if the feeder numbers were 0 in all cases.

[T3P3]

Need to think about how material runout failover works between tools (e.g. with a tool changer having two tools and the print job continuing with the second tool when the first tool runs out)

Consider how a filament management system inactive slot or inactive tool is reloaded while the print continues.

consider using a general mechanism that handles feeders or tools for failover.

[T3P3]

Some interaction with #822 as far as supporting how an filament management system works.