Support nested structures in import command

[lawrencejones]

When running an import on source data that is nested, we will fail. This is because we try shoving whatever we find at the top-level key of any entry into the attributes and that's not going to work that well.

We should:

1. Recurse into the entry structures so we discover child keys, in a similar way as we do in https://github.com/incident-io/core/pull/11248
2. Correct type the resulting catalog attribute from what we find in the entry (right now everything is a string)

I've opened this issue because the customer requesting the import command is going to be using a CSV, and that happens to mean we encounter neither of these problems. Figure we can leave this open until someone genuinely needs it.