Json: map non-primitive collections to JSON objects

[maumar]

Currently collections are mapped to JSON arrays, we could map them to JSON objects instead.

[roji]

@maumar can you provide some context on what this means?

[AndriySvyryd]

This would allow to map collections of owned types as objects where one of the properties on the owned type is mapped to the object key and the value contains another JSON object containing the rest of the properties.
In the client side it would still be a regular collection, as opposed to #21262