SectionConfig: fix handling unknown sections

SectionConfig: fix handling unknown sections

if we're parsing an unknown section, we cannot check the schema with
`is_array` to check if it's an array type or not, thus we have to
handle that separately.

fix this by handling data in unknown sections like an array similar to
"cb2646c7b4974e33f4148752deec71f0d589b0f3" in proxmox-section-config.
This way we can write unknown section out again like we parsed it.

Add a regression test for an unknown field not in the schema.

This fixes an issue, where calling `qm destroy ID --purge` removed much
of the configs ob backup jobs (since there we parse an 'unknown' section
and run into the `is_array` error)
(Reported in the forum: https://forum.proxmox.com/threads/132091)

Suggested-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>