feat: init SwiftUI library for FirebaseAuthSwiftUI

[russellwheatley]

Work in progress, no functionality at the moment. Just setting up the project:

• Setup a separate directory for SwiftUI libraries.
• Created two libraries; FirebaseAuthSwiftUI (this has some basic types/class which will be needed later) & FirebaseAuthEmailSwiftUI for email provider functionality.
• Created a SwiftUI example app in samples/swiftui which pulls in the above libraries for developing.

[morganchen12]

Overall looks good. Had one architecture question for consideration.

[morganchen12]

Since we're targeting this toward SwiftUI users, what do you think of adding a @StateObject that tracks the auth state so consumers can use it directly instead of having to set up their own wrapper? This could be the auth provider itself or a separate object that's managed by the auth provider, or a default implementation of the AuthListenerProtocol.