refactor: follow up PR with updates based on feedback

[russellwheatley]

Contains the following:

1. Refactored the protocol and class names for providers so they contain AuthUI. Example - FacebookProviderSwift has changed to FacebookProviderAuthUI.
2. Updated Button Views with label argument rather than using anonymous closure.
3. Implemented ContentView for SwiftUI example app.
4. Created Previews for almost all Views, needed a FirebaseApp configured for a lot as we initialise FirebaseAuth in AuthService. I didn't create a preview for AuthPicker as it will be refactored soon with the implementation of View modifiers to choose authentication methods available. I will add it once that has been refactored.

Let me know what you think, @peterfriese. Just as an aside, the UI Views don't look great at the moment as we're awaiting UI design.

[peterfriese]

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