Use the new koschei-messages schema package

[abompard]

This change will make use of the new schema package, which will let FMN produce notifications for users.

Ref: https://github.com/fedora-infra/fmn/issues/915

Caveats:

• The schema package is not in Fedora yet, it's installed from PyPI for now.
• There is currently no schema for the messages sent by the OSCI plugin, but it's not as urgent as they are not as directly useful for user notifications.

[mizdebsk]

This change is now deployed in staging at https://koschei.stg.fedoraproject.org/ Example message emitted by Koschei: https://apps.stg.fedoraproject.org/datagrepper/v2/id?id=7f1d6e43-ff08-4180-a3cd-024cf7ac3d1b&is_raw=true&size=extra-large