Add possibility to use stablecoin pools via vaultmaxi

[kuegi]

Goal:

If stablecoin pool is defined as poolpair, maxi takes DUSD loan and pairs it with the stablecoin which needs to be in the collateral.

Future version might add an automatic swap of DUSD to the stablecoin if not enough is in the collateral. This version will behave exactly like the DUSD-DFI single-mint case.