Add registration token flow type

[robbystk]

Why?

• I want the Element iOS app to support registration tokens which it currently does not
• Add support for Token Authenticated Registration (MSC3231) element-hq/element-ios#6507
• MSC3231: Token Authenticated Registration

⚠️ Warnings:

• I do not yet have a way to run the tests or even compile this code so this change is COMPLETELY UNTESTED
• This is my first time writing Swift or Objective-C
• I probably missed some other places in the code that need to be changed

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off
  • (see commit messages for sign-off)