Add configuration schemas

[maxfischer2781]

This PR adds schemas for configuration validation.

• plugins can define schemas
• configuration is verified by schema if defined
• errors report useful information on the config
• cobald configuration defines a schema
• documentation on how to define a schema
• change fragment

Also adds documentation to most of the existing configuration backend. Hopefully, this makes it slightly less arcane.

Closes #84.

[maxfischer2781]

@giffels The current plan is to have the cobald.daemon.plugins.constraints define a (pydantic) schema for the entire plugin:

@plugin_constraints(required=True, schema=cobald_schema)
def load_pipeline(content: list):
    ...

It might be useful to allow schemas for yaml_constructors as well – that would make it easier to gradually add schemas "inside" a config, and free the plugin-wide schema from covering every case. Should I add it as well?

[maxfischer2781]

Closing this for the time being. With the current feature set, there isn't really any advantage over plugins just doing their own validation. Adding more meaningful features (e.g. precise configuration warnings) would be a too significant change right now.

This should be worth to revisit once the core library has been cleaned up and streamlined more.