Add support for nested fields

[marangonico]

Updated facets, filters, and aggregations to handle nested fields, enabling more flexible query structures. Adjusted logic to check for "nested" definitions and construct queries accordingly, ensuring compatibility with both standard and nested data structures.

This PR improves the handling of nested fields.

Let's take category as an example.
The following snippet shows how to add a facet for the category id.
A new dictionary key "nested" is needed with the "path" to be used.

OSCAR_ELASTICSEARCH_FACETS = [
    {
        "name": "categories.id",
        "label": "Category",
        "type": "term",
        "nested": {
            "path": "categories",
        }
}