Validate and process fragment input parameters on backend

[DeamonDev]

💥 Proposal

Currently the typing part of fragment input parameters are processed on frontend. The model definition is fetched and frontend sends information about classes obtained from this model definition information and provided user input. I propose to change this and let frontend only pass raw strings data which will be validated/processed based on model definition on the backend.

This solution will also simplify creation of the parser for arbitrary-depth manually passed user types like List[List[List[T]]].