[link-quality] introduce RssAverager and keep track of average RSS for a received message (#1961)

This commit  introduces a new class `RssAverager` (which contains the
existing averaging logic from`LinkQualityInfo` class). It also updates
`Message` class to maintain the average RSS for a received message
(note that a message may be composed of multiple 802.15.4 fragments
data frames each received with different signal strength). The
`MeshForwarder::LogIp6Message()` is updated to log the RSS value for
received/dropped messages.

src/core/api/message_api.cpp

@@ -87,6 +87,12 @@ void otMessageSetDirectTransmission(otMessage *aMessage, bool aEnabled)
     }
 }
 
+int8_t otMessageGetRss(otMessage *aMessage)
+{
+    Message *message = static_cast<Message *>(aMessage);
+    return message->GetAverageRss();
+}
+
 otError otMessageAppend(otMessage *aMessage, const void *aBuf, uint16_t aLength)
 {
     Message *message = static_cast<Message *>(aMessage);