Simplify support of username as a nested property

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).

Start by tracing how Spring Security handles the User Info response and the user-name-attribute setting. Reproduce the nested data.username example, then define and implement the chosen approach so a nested username can be used without the current extra work.