[json] parse depth limit

[segoon]

Parsing depth should be limited to some sane value, e.g. 128. Too big value may lead to a recursive call to rapidjson which might lead to stack overflow.

Internal ticket: 7801

[Anton3]

This can be implemented without patching rapidjson itself. formats::json::FromString can use a wrapper class/struct around impl::Value with rapidjson visitation API that keeps track of the depth and forwards to impl::Value's visitation API.

[moki]

@segoon can you please assign this one to me?