Add `source_file` to `incus_image`

[stgraber]

This new image source should take both a metadata_path (optional) and data_path variable which points to image file(s) on the user's system.

The image will then be created by pushing those files to the server in the same way that's done by incus image import.

[breml]

Example:

resource "incus_image" "from_file" {
  source_file = {
    data_path     = "/some/dir/file"
    metadata_path = "/some/dir/metadata"
  }
}

resource "incus_instance" "test1" {
  name      = "test1"
  image     = incus_image.from_file.fingerprint
  ephemeral = false
}

[breml]

@stgraber I understand that the data_path would be used for two different things depending on metadata_path being provided or not. If metadata_path is not provided, we expect data_path to point to an unified image. In the other case, if metadata_path is provided, we expect data_path to point to a rootfs tarball, correct?

[stgraber]

Yep, that's correct