Create serialized generic subclasses automatically

[bddckr]

This is useful for the boilerplate that is UnityEvent<T> subclasses everywhere as well as unsupported types the user defines as well as Dictionary<TKey, TValue> 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.