Add a "filter" (search) endpoint that can be used by Orthanc servers to determine if a user or group exists

The Orthanc local ACL policy system requires that Orthanc be able to query Sonador to determine if users or groups exist, and whether the user has access to a specific Orthanc instance. It is further needed for retrieving user details for comments and other views. For example, the comment view needs to determine if a user making a request to modify a comment is the original author of that comment. If not, the request to update the comment should be denied.

Needed endpoints:

• Query/search for a single user
• Query/search for a single group
• Batch user retrieval: fetch details for a group of users by primary key/UID (31ae1533 and sonador-client@e8dfcd3d)
• Batch group retrieval: fetch details for a collection of groups by primary key/UID (31ae1533 and sonador-client@e8dfcd3d)
• Add a "unified search" endpoint that allows for both users and groups to be searched from the front-end (4d545cdc and sonador-client@55bdff20)