Migrate iOS SDK to SDK style project

[frederikprijck]

Changes

Migrates the iOS SDK to be an SDK style project to allow for targetting multiple platforms (Xamarin and .NET iOS).

Testing

Please describe how this can be tested by reviewers. Be specific about anything not tested and reasons why. If this library has unit and/or integration testing, tests should be added for new functionality and existing tests should complete without errors.

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

Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors
• [x] All code guidelines in the CONTRIBUTING documentation have been run/followed

[frederikprijck]

Closing in favor of https://github.com/auth0/auth0-oidc-client-net/pull/275