Create serialized generic subclasses automatically

This is useful for the boilerplate that is `UnityEvent` subclasses everywhere as well as unsupported types the user defines as well as `Dictionary` for example.

The documentation should give a quick hint (some links?) on how to ensure the type is serializable as just making non-generic subclasses types for types like `Dictionary` doesn't make them magically serializable. At least `ISerializationCallbackReceiver` should be linked.