LXD backend: allow nestable containers

Since LXD allows [nested containers](https://documentation.ubuntu.com/lxd/stable-5.21/reference/instance_options/#instance-security:security.nesting), it would be nice to be able to declare that the LXD backend should run its containers with nesting enabled.

Start by locating the LXD backend entry point and its container configuration path, then read the linked LXD instance options documentation for security.nesting. Trace how declared container options reach launched instances and verify that nesting can be enabled through the backend's existing configuration and tests.

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