Json: move SqlServerValueGenerationConvention.GetValueGenerated(IConventionProperty property) JSON logic to relational

[maumar]

Currently we handle value generation in sql server. This is because in JSON case (if entity has been marked as json in the model configuration) we don't get to relational at all - property.GetMappedStoreObjects(StoreObjectType.Table) returns null because it relies on getting a column name for the property and for JSON that always goes back as null. Ideally we would refactor that code, so that correct table is returned and we get to relational so we can set value generated for JSON all in one place