Add programmatic configuration schemas

[maxfischer2781]

The configuration is currently treated as a blob of raw YAML/JSON equivalent data. This makes validation and error reporting difficult, as keys/values are checked on usage.

cobald should provide means to define expected schemas for configuration. This would allow to efficiently detect and report errors. Schemas would also allow to generated documentation; this is not required for the base functionality.
Schemas perhaps should (allow to) be automatically generated from the objects taking the configuration.

Related issues:

• Improve configuration management tardis#152 motivates this issue, but functionality is not plugin specific.
• Nicer YAML configuration error messages #54 on pretty configuration error reporting

External packages:

• The pydantic package for validating runtime types based on annotations.