Allow craft tools to function inside a LXD parent container when performing a LXD managed build

[flotter]

What needs to get done

Please refer to this: https://ubuntu.com/blog/nested-containers-in-lxd

Currently when using spread with a LXD backend, running a craft tool does not appear to be supported using a nested LXD managed build backend. If this is actually supported, then this request is not needed ?

Supporting nested LXD instances in unprivileged containers require both the LXD parent and the LXD child container to have some agreements with regards to idmap ranges.

This feels like a great feature than can be solved inside the common craft libraries, with a documented requirement for the LXD parent container to support this usage case.

Here are some resources on the topic:
https://documentation.ubuntu.com/lxd/en/latest/userns-idmap/

Why it needs to get done

This would expand the places where craft tools would "just work" without requiring provider specific tweaks. One use case is Spread LXD backend running craft tools suing managed builds.

[mr-cal]

This has a similar underpinning to UID configuration as canonical/craft-providers#259. Neither craft-application nor craft-providers exposes these settings for UIDs.

Can you specify what workarounds you did? I'm wondering if this can be fixed in the set up of the outer lxd instance.