Recreate SCIM::Users collection entity

Problem

Move back to using a collection entity for SCIM::Users to group behaviour for presenting multiple results along with a startIndex, count, itemsPerPage and a schema.

Background

Before !14892 (merged) we took this approach but were only presenting an individual result. Getting this to work with multiple results was problematic and so we took the approach of using multiple present statements to build a hash instead, as suggested in https://github.com/ruby-grape/grape/issues/241.

• @smcgivern started a discussion: (+3 comments)

  Why did we stop using an entity for the top-level response? Is it because we needed to mix in some params?

  If so, I think it would be good to create follow-up issue (not high priority) to put this back in an entity in future.