Provide a default user service

[kbarbounakis]

Description
@themost/data does not have an application user service in order to get users and roles. It uses default services to get user and group data e.g. context.model('User').where('name').equal('user1').getItem()
This operation is completely safe but sometimes it's better to have a single operation to get user or group data. This would be very useful for services like DataPermissionEventListener in order to reduce code.

Solution
A UserService or UserProvider should be suitable to resolve this issue.

class UserService {
     getUser(name) {
          //
     }
     getGroup(name) {
        //
     }
     ...
}

Alternatives
No

Additional context
No