Improve custom JsonConverter of IData

[simon-wacker]

Our JSON converter implementation https://github.com/ise621/metabase/blob/110b2e0fec32642476ea39bcab6f60182a87dc8a/backend/src/GraphQl/Databases/DatabaseResolvers.cs#L250 is not generic, inspired by https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?pivots=dotnet-5-0#support-polymorphic-deserialization and https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?pivots=dotnet-5-0#an-alternative-way-to-do-polymorphic-deserialization and seems quite brittle to me. It woud be great if some C# GraphQL framework supported polymorphic types. There is a pending pull request for the .NET GraphQL Client with such a converter.