Json/Migrations: flow the json type mapping (or relevant type mapping info) to column in the model differ so we can create correct default value and clr type for json column operation

For now the clr type (and default value) are hard-coded. It's not a major issue as we can't really do anything intelligent here, apart from creating a string representation for empty json (which won't work if the model requires an entity there), but it will be "more correct" way of doing things