Add schemas for `octue.yaml` and `app_configuration.json`

[cortadocodes]

There's currently no validation of octue.yaml and app configuration files, meaning they can be messy and invalid, and cause unforseen problems. We should add schema for them (hosted in the Strands app) so users don't have to introspect octue.configuration.ServiceConfiguration to work out what keys/values are needed or allowed.

We could potentially use this: https://github.com/keleshev/schema