Add the ability to use the expand parameter for the user resource in Rest API

Problem Definition

In Confluence Cloud, it's possible to use the expand parameter to get further information about a user. For example, something like:

/rest/api/user?username=(usernamehere)&expand=details.personal

will return information like the email address. This does not work in Server.

Suggested Solution

Add this parameter for the user resource.

Workaround

None at this time.