Allow configuring debootstrap implementation, add mmdebstrap support

Adds mmdebstrap-specific code to bootstrap_debootstrap, and allows the user to set whether they want to use debootstrap or mmdebstrap to bootstrap the chroot. Other debootstrap alternatives could be supported fairly easily by adding the appropriate logic for those frontends to bootstrap_debootstrap.

Closes: #1031929

(Note: I did NOT try building an arm64 ISO or using the foreign architecture bootstrapping features of live-build. The amd64 code could also use a bit more testing since I didn't test these changes in isolation yet.)