[udp] expose an API to send UDP datagram (#3822)

This commit exposes an API to send UDP datagram, this is usefully when the
UDP receiver handler needs to send messages.

include/openthread/udp.h

@@ -97,6 +97,19 @@ otError otUdpAddReceiver(otInstance *aInstance, otUdpReceiver *aUdpReceiver);
  */
 otError otUdpRemoveReceiver(otInstance *aInstance, otUdpReceiver *aUdpReceiver);
 
+/**
+ * This function sends a UDP message without socket.
+ *
+ * @param[in]  aInstance     A pointer to an OpenThread instance.
+ * @param[in]  aMessage      A pointer to a message without UDP header.
+ * @param[in]  aMessageInfo  A pointer to a message info associated with @p aMessage.
+ *
+ * @retval OT_ERROR_NONE     Successfully enqueued the message into an output interface.
+ * @retval OT_ERROR_NO_BUFS  Insufficient available buffer to add the IPv6 headers.
+ *
+ */
+otError otUdpSendDatagram(otInstance *aInstance, otMessage *aMessage, otMessageInfo *aMessageInfo);
+
 /**
  * This callback allows OpenThread to inform the application of a received UDP message.
  *

src/core/api/udp_api.cpp

@@ -150,3 +150,11 @@ otError otUdpRemoveReceiver(otInstance *aInstance, otUdpReceiver *aUdpReceiver)
 
     return instance.Get<Ip6::Udp>().RemoveReceiver(*static_cast<Ip6::UdpReceiver *>(aUdpReceiver));
 }
+
+otError otUdpSendDatagram(otInstance *aInstance, otMessage *aMessage, otMessageInfo *aMessageInfo)
+{
+    Instance &instance = *static_cast<Instance *>(aInstance);
+
+    return instance.Get<Ip6::Udp>().SendDatagram(*static_cast<ot::Message *>(aMessage),
+                                                 *static_cast<Ip6::MessageInfo *>(aMessageInfo), Ip6::kProtoUdp);
+}