Rewrite how admin elements are created

[szymach]

Instead of manually creating element classes for every element, they could be composed of building blocks that could come from various sources. For example, elements without any additional logic could be nothing more than an entry in a YAML file, which would only provide element/Datagrid/Datasource id and a form class. If there was need for custom logic for ex. Datasource, a microservice class with just the __invoke method could be created to handle that one aspect. This way we could reduce 90% of the generated code.