Expose Biometrics class through Swift for Swift Package Manager

[KaneCheshire]

Currently, if you're using TABTestKit with Swift Package Manager, to use the Biometrics class functions directly you need to import Biometrics in the file, because of the way the package has to be split up into targets for SPM support.

I think we can improve this, by creating a wrapper class in the Swift target that exposes the ObjC code. If we keep the name and function signatures exactly the same, it shouldn't be a breaking change for anyone. Would be super great if we could completely hide the ObjC code, which is slightly more complicated so you might need to tap up someone a bit more seasoned with C headers if you pick this up and not sure what to do.

[KaneCheshire]

Remember to update the README when this is done, since it references this issue directly in more than one place :)