Added support for ephemeral sessions [SDK-1559]

[Widcket]

Changes

• Added a new public method useEphemeralSession() to WebAuth that sets prefersEphemeralWebBrowserSession to true on iOS 13+ and macOS. It has no effect on older versions of iOS. As it depends on prefersEphemeralWebBrowserSession, this method is only available for Swift 5.1+. Usage of this method disables SSO.
• Renamed the file WebAuth.swift to WebAuthenticatable.swift.
• Renamed a couple of parameters in internal methods.

Testing

This change was tested manually by performing login and logout on macOS 10.15.4 and iOS simulators 10.3.1, 11.4 and 13.3.

• This change adds unit test coverage
• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines
• All existing and new tests complete without errors
• All active GitHub checks have passed

[cocojoe]

Seems like a lot of refactoring going on for what is basically surfacing this ephemeralSession flag.

[Widcket]

Seems like a lot of refactoring going on for what is basically surfacing this ephemeralSession flag.

I always try to leave the codebase in a better shape than before. It's better to do that incrementally like this.