Implement same network config validation and warnings as incus-os image-customizer #998

We've seen instances of users using the wrong network config format when downloading ISO images from Operations Center, mainly having the network config nested in the config block.

image-customizer has a few preventive measures:

• There is validation on the network config block:
  • incus-osd/image-customizer: Reject network seeds with a "config" key lxc/incus-os#1264
• The UI containers a warning:
  • https://github.com/lxc/incus-os/blob/b902209aafbcf071a9fc387ac056c0a58f36d731/incus-osd/cmd/image-customizer/html/index.html#L342

It would be good to have the same measures in Operations Center.