Feature Request: Re-implement the LXD build environment

[林博仁(Buo-ren Lin)]

In legacy Snapcraft, it is possible to set `SNAPCRAFT_BUILD_ENVIRONMENT=lxd` and Snapcraft will automatically create a `snapcraft-_snap_name_` LXD container to build the snap, however, this feature is dropped in Snapcraft 3.0. I would suggest re-implementing this feature again as it is useful for emulating a cleanbuild and the current default `multipass` build environment has its own limitations(not suitable for building snaps that requires lots of primary memory, etc.)

[Sergio Schvezov (sergiusens)]

This is coming back in a different form, tracked under LP: #1805221