Replace JSONObjectPayload with Codable on Credentials [SDK-2958]

[Widcket]

Changes

⚠️ THIS PR CONTAINS BREAKING CHANGES

This PR replaces the conformance of Credentials to JSONObjectPayload with conformance to Codable and uses the newer, non-deprecated methods of NSKeyedArchiver and NSKeyedUnarchiver to archive and unarchive Credentials.

Additionally, the Credentials class was made final.

Care was taken to ensure that credentials archived with Auth0.swift 1.x can be unarchived successfully, and the other way around (in case customers need to downgrade to 1.x). Otherwise, if apps are using the Credentials Manager of Auth0.swift 1.x and then get upgraded to 2.x, the stored credentials will not be readable, essentially logging out all the users.
To maintain compatibility, @objc(A0Credentials) was added back to Credentials, which still needs to inherit from NSObject.

Dropping NSSecureCoding in favor of Codable + the Codable-supporting methods of NSKeyedArchiver and NSKeyedUnarchiver has been ruled out for the sake of this compatibility after doing a spike testing it.

Testing

The backward compatibility of the archived credentials was tested manually in a test app, by:

• Storing credentials with the Credentials Manager from Auth0.swift 1.38.0 and then reading them back using the Credentials Manager from this PR.
• Storing credentials with the Credentials Manager from this PR and then reading them back using the Credentials Manager from Auth0.swift 1.38.0.

These tests were performed on both iOS 15 (simulator) and macOS Big Sur.

• 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]

Re-added for backward compatibility of archived credentials.