Allow `lxc init/launch` of an instance where the source image is in another project

[markylaing]

As a user with access to project A and project B, I should be able to launch an instance in project B while specifying an image in project A.

For example, if I have an image with alias my-image in project A, and I want to create an instance my-instance from it in project B:

lxc project switch B
lxc launch <image-fingerprint-or-alias> <image-name> --image-project A

Additionally, this should still work if the user does not have access to project A if the image is public.

[tomponline]

@markylaing see also https://github.com/canonical/lxd/issues/10089