Add support for async/await [SDK-2981]

[Widcket]

Changes

This PR adds support for async/await to Request, Web Auth, and the Credentials Manager.

• In the case of Request, an async overload of start() that returns the response was added.
• For Web Auth, an analogous async overload of start() that returns the credentials was added, along with an async overload of clearSession().
• For the Credentials Manager, async overloads were added for the methods credentials() and revoke().

Testing

• Unit tests were added for the Request the Credentials Manager overloads.
• The overloads for Web Auth and the Credentials Manager (the credentials() method) were tested manually in a test app on iOS 15.2 (simulator) and macOS 11.6.1, using Xcode 13.2 beta 2 (13C5081f).
• Additionally, the Request overload was tested manually in a test app on tvOS 15.2 (simulator) and watchOS 8.3 (simulator), using Xcode 13.2 beta 2 (13C5081f).

• 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

[Widcket]

For macOS BigSur (11.x) with Xcode < 13.2 (swift < 5.5.2).