Add all projects support to fetch images

[edlerd]

Required information

• Distribution: snap
• Distribution version: 6.1

Issue description

Add a way to fetch images from all projects. Suggesting a new param: GET 1.0/images?all-projects=true. A similar parameter exists on the instance endpoint.

Motivation: We need all images for the permission selection in LXD-UI. We present the list of images to create a permission, and the auth API only gives us fingerprints for the images. So we'd like to fetch all images to show the nice names. This currently only works for images in the default project, others stay with the fingerprint, as we can't load the images for all projects.