Add Purge Information to Spoolman

[jeffmccollum]

By calibrating the amount of purging using https://www.printables.com/model/536401-bambu-lab-ams-purging-strips-p1p
It's possible to save quite a bit of filament on large multicolor prints. The downside to this is maintaining a list of every filament combination and it's purging amount.
There is https://github.com/RussHewgill/purgedb which does this however it's a desktop app and is another system/database to keep a list of filament in.
If Spoolman could add the ability to link two filament types together, with an entry for the purge amount this would be great as Spoolman is already tracking filaments.

I know custom fields can be added to the filaments, but I'm not sure how well that could scale for x * x filament combinations.