Out of curiousity, what else would be needed for other kinds of extensions?

This is basically a copy of https://github.com/flatpak/flatpak/blob/main/app/flatpak-builtins-build-export.c#L676 - so I'd like to see the following:
* A link to the original
* An explanation of why the duplication is needed (we don't run build-export for runtimes and now extensions) [Hmm, I think the reason for runtimes is mostly for efficiency - to avoid unpacking the huge tarball, and then reimporting it into ostree. But maybe there were other reasons as well ]
* An explanation of why the full checks in build-export aren't duplicated here - e.g. uri is required, name can't contain a /.
* A note about the fact that build-export support multiple Extra Data sections, but only one is allowed here

This needs to be renamed to _create_runtime_or_extension_oci()

I don't know extension building very well, but this looks generally good. Do you have an example somewhere of how it is used - how an extension Flatpak will look?

ping @kalev

ping @kalev @otaylor

New PR for extensions without extra-data: #45

I've borrowed part of this for a different implementation in https://gitlab.com/flatpak-container/flatpak-container-tools/-/merge_requests/13

Closing in favour of #45