help serde disambiguate between numeric quantities and "freeform string"

I don't love this, but i couldn't figure out how to tell TOML's
Deserialize to accept an enum of a usize or a string for `quantity`.

modular-recipes/recipes/peristaltic-pump/peristaltic_pump.toml

@@ -6,7 +6,7 @@ tube_support = {quantity=1}
 
 [inputs.silicone_tubing]
 type = "silicone tubing, 6mm OD, 4mm ID"
-quantity = "1 meter"
+amount = "1 meter"
 comment = "PVC tube isn't flexible enough.  Adjust quantity to suit your application."
 
 [inputs.m4_25_bhcs]