[enhancement]: Mechanism for custom config modules to define schema-complaint keys

# Enhancement

There is no way for custom modules to extend cloud-init's cloud-config jsonschema. If they operate with new unknown keys, cloud-init's jsonschema validation is going to complain about it.

Provide a mechanism to allow custom modules to define their keys jsonschema-wise.

## Possible approaches

1. Provide a namespace where the config keys aren't validated. This is related to https://github.com/canonical/cloud-init/issues/5439's proposal.
2. Load custom json-schema definitions and inject them in cloud-init's cloud-config jsonschema.