Simplify support of username as a nested property

[rwinch]

For Spring Boot users who want to change the property of the User Info response that is used to create a username, they can easily change this by specifying the user-name-attribute. However, if the username is nested this does not work. For example, X wraps responses in a data property which means the username is data.username

{
  "data": {
    "id": "2244994945",
    "name": "TwitterDev",
    "username": "Twitter Dev"
  }
}

Unfortunately, this currently requires quite a bit more work than a non-nested property.

We could consider a way to access nested properties or to simplify transform the response so it is not nested (probably preferred).