Make the bulk user rest api available for Confluence Cloud

Summary

As of the GDPR changes, vendors are making some extra requests to the user api.
In Jira, the bulk user api (https://developer.atlassian.com/cloud/jira/platform/rest/v2/#api-api-2-user-bulk-get) is a big improvement on this.

Vendors would like to achieve the same in our Confluence application as well, as it would increase the performance of their application, and decrease the number of requests made to Confluence.