Support C# union mapping to JSON

[AndriySvyryd]

JSON $type property would be used as the discriminator.

Note that union types are structs, so they will have the same limitations in EF as other structs.

Related to #36375

[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• Support inheritence with JSON POCO mapping #27779
• 🐛 EF Core 9: .ToJson() doesn't serialize polymorphic types with [JsonDerivedType] attributes #36450