Support serialization key transformer

[MP0w]

In swift code we usually use camelCase while for JSON other convention (mostly snake_case) are used, I would don't like forcing the user to name the property we might have easy ways to support this:

Detailed design

SnakeCaseTransformer(User()) // convert the serialized object to snake case

This can be done by using CustomSerializable.
It would be good if the transformed can also be used to rename properties of an entity:

struct User: Serializable {
   let id: Int
}

// you want id (Storable) to be named `userId` in the JSON

protocol KeyTransformer: CustomSerializable {
    func transformedKey(for candidate: String) -> String
}

extension KeyTransformer {
// default implementation that use `transformedKey...`to transform the keys
}