Allow for attribute filter for UAA users

[drennalls]

https://docs.cloudfoundry.org/api/uaa/version/4.6.0/index.html#list-with-attribute-filtering allows for querying UAAs /Users endpoint with filters based on attributes. However I don't see a way to make use of it via the cf-java-client API for UAA, based on ListUsersRequest
Is that possible now (and I'm just missing something) or planned for future ?

[twoseat]

This issue is on hold pending the outcome of https://www.pivotaltracker.com/n/projects/997278/stories/152242975 - as soon as that's resolved I'll add the feature to the client.