Addition of image requirements metadata

[cavcrosby]

@stgraber per our discussion. My implementations stick mostly to what you suggested but there are some subtle differences. I think documentation is at a good start.

Closes #9408

[lxc-jenkins]

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[lxc-jenkins]

Documentation preview available at: https://linuxcontainers.org/lxd/docs/pr.9827/

[stgraber]

Looks good, just a few tiny tweaks.

I think we should also add an API extension for this. For that, add an extra commit titled api: image_restrictions and have that contain an addition to both:

• doc/api-extensions.md
• shared/version/api.go

[stgraber]

Thanks!

I'll do a quick review again, then will rebase to fix that conflict and merge your fixes into their respective commits.
Then just wait for Jenkins to give the all good and this can get merged!

[stgraber]

jenkins: test this please

[stgraber]

jenkins: test this please

[stgraber]

Thanks!

[lxc-jenkins]

Testsuite passed