Feature: Add method to get all subclasses of a class in schema registry

[TeresiaOlsson]

Description, motivation and use case

It would be useful with a method in the SchemaRegistry to list all subclasses of a class which are available in there registry.

This can for example be used to improve the user-friendliness of this example for how the ConfigurationSchemas in the registry can be used to program the configuration for those who wish: https://python-accelerator-middle-layer.github.io/documentation/how-to/configuration/use-configuration-schema.html

Proposed solution
Add a subclass_of method.

Checklist

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