Allow LXD profiles to be passed when launching

When launching a series of tasks on LXD backends it would be ideal if we could also passing in a series of profile names to also be used when launching[1]. This would allow for things like nesting or privileged containers.

1. https://github.com/snapcore/spread/blob/c85b89fe2746a509b2b9f28be5c4dce1130d86f1/spread/lxd.go#L110

Start in spread/lxd.go around the launch code at the referenced line and trace how LXD task launches are assembled. Add support for receiving the requested profile names and applying them during launch, then verify that launches can use profiles for cases such as nesting or privileged containers.

Written by the indexing model from the issue text.