owm.schema: initial proposal

[SvenRoederer]

This proposal is based on the notes that have been made in

• the Pad (https://pad.freifunk.net/p/owm-datastructur)
• Freifunk-Mailinglist (https://lists.freifunk.net/pipermail/wlantalk-freifunk.net/2021-July/001352.html)

It's expected to receive some changes.