feat: create instance from backup file [WD-13919]

[mas-who]

Done

• Allow instance creation from file import

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @mas-who or @edlerd for access.
   • With a local copy of this branch, build and run as described in the docs.
2. Perform the following QA steps:
   • Run lxc export <instance> <filename> to export an instance
   • Navigate to Create Instance form
   • Click "Import from file" button
   • Select the exported instance backup file
   • Set an instance name and select a target storage pool
   • Create the instance

[webteam-app]

Demo

Jenkins

demos.haus

[mas-who]

Note: I didn't add the target cluster member selector as lxc export does not support the --target flag. This is quite interesting since the it is calling the same API endpoint. Sending a request with the target query parameter has no effect.

[mas-who]

@edlerd made all the requested changes, ready for another pass thanks :)