add ldap user filter

ee/lib/ee/gitlab/ldap/user_filter.rb

+module EE
+  module Gitlab
+    module LDAP
+      class UserFilter
+        def self.filter(*args)
+          new(*args).filter
+        end
+
+        def initialize(proxy, filter)
+          @proxy = proxy
+          @filter = filter
+        end
+
+        def filter
+          @proxy.adapter.ldap_search(options).map(&:dn)
+        end
+
+        private
+
+        def options
+          { base: config.base, filter: construct_filter }
+        end
+
+        def construct_filter
+          Net::LDAP::Filter.construct(@filter)
+        end
+
+        def config
+          @proxy.adapter.config
+        end
+      end
+    end
+  end
+end