[message-queue] enable adding a message to head of queue (#2246)

This commit updates the `MessageQueue` implementation to allow
messages to be added to head/front of the queue. It adds new methods
to the `MessageQueue` class and a corresponding one in public
OpenThread APIs (`otMessageQueueEnqueueAtHead()`). This commit also
updates the `test_message_queue` unit test to add cases/scenarios for
testing the new methods and public APIs.

src/core/api/message_api.cpp

@@ -123,6 +123,13 @@ otError otMessageQueueEnqueue(otMessageQueue *aQueue, otMessage *aMessage)
     return queue->Enqueue(*message);
 }
 
+otError otMessageQueueEnqueueAtHead(otMessageQueue *aQueue, otMessage *aMessage)
+{
+    Message *message = static_cast<Message *>(aMessage);
+    MessageQueue *queue = static_cast<MessageQueue *>(aQueue);
+    return queue->Enqueue(*message, MessageQueue::kQueuePositionHead);
+}
+
 otError otMessageQueueDequeue(otMessageQueue *aQueue, otMessage *aMessage)
 {
     Message *message = static_cast<Message *>(aMessage);