Add support for transformable property types

[admkopec]

This PR adds support for @Attribute(.transformed(by: Transformer.self)) property as per issue #4. This only adds support for type specified Transformers inheriting directly from ValueTransformer.

Due to the need of registering the transformer and issues with NSClassFromString() the named equivalent of this attributed was not implemented as part of this PR.

Additionally when testing the implementation, I encountered an error with CodableBox transformer which could be passed a base type T as the value. This was addressed with additional check. Not sure if the reverse transformer should return value of type T or CodableBox then? Although the committed approach with base type T appears to work with my tested model.

Additionally, while adding tests for Transformable properties, I added a test case for RawRepresentable enums.

[helje5]

Thanks! Will have a look. It seems to reverse the fix I made for Codable structs (where the valueType setter was overwritten by the options), but I only had a quick look.

[helje5]

Hm, if we can store the Codable value as-is, we shouldn't even need the CodableBox, right? The Transformer could directly do the coding from/to Data 🤔

But I don't understand how you ended up w/ the base value in the Transformer?

[helje5]

I played w/ that, and it looks like we don't need the CodableBox but just can stick the Swift value into the dynamic property? Not sure whether this is going to have unwanted side effects :-)
branch feature/no-codable-box-1