[ICU-621] Add ExperimentalGroupUnreadChannels setting (#8127)

* Add ExperimentalGroupUnreadChannels setting

* track ExperimentalGroupUnreadChannels in diagnostics

model/config.go

@@ -214,6 +214,7 @@ type ServiceSettings struct {
 	EnablePreviewFeatures                             *bool
 	EnableTutorial                                    *bool
 	ExperimentalEnableDefaultChannelLeaveJoinMessages *bool
+	ExperimentalGroupUnreadChannels                   *bool
 	ImageProxyType                                    *string
 	ImageProxyURL                                     *string
 	ImageProxyOptions                                 *string
@@ -422,6 +423,10 @@ func (s *ServiceSettings) SetDefaults() {
 		s.ExperimentalEnableDefaultChannelLeaveJoinMessages = NewBool(true)
 	}
 
+	if s.ExperimentalGroupUnreadChannels == nil {
+		s.ExperimentalGroupUnreadChannels = NewBool(false)
+	}
+
 	if s.ImageProxyType == nil {
 		s.ImageProxyType = NewString("")
 	}