Support "recursive" object definition in the jsonschema UI builder

[rubenfiszel]

Most payload have deeply nested structure and we should not assume the input is a flat object, so the UI builder should offer a convenient interface to define deeply nested object's jsonschema.

[axelbdt]

I have made some progress with this one. The set of changes is growing so I think it would be nice to discuss it with the team to get earlier feedback on style and design.

The code is on my exploration branch: https://github.com/axelbdt/windmill/tree/recursive_schema_ui_expoloration

[rubenfiszel]

Done