[mle] add discovery request callback (#5131)

This commit allows notifying the application a discovery request is
received, which is useful if the application wants to be more active
when a joiner comes.

include/openthread/instance.h

@@ -53,7 +53,7 @@ extern "C" {
  * @note This number versions both OpenThread platform and user APIs.
  *
  */
-#define OPENTHREAD_API_VERSION (22)
+#define OPENTHREAD_API_VERSION (23)
 
 /**
  * @addtogroup api-instance

include/openthread/thread.h

@@ -805,6 +805,37 @@ void otThreadRegisterParentResponseCallback(otInstance *                   aInst
                                             otThreadParentResponseCallback aCallback,
                                             void *                         aContext);
 
+/**
+ * This structure represents the Thread Discovery Request data.
+ *
+ */
+typedef struct otThreadDiscoveryRequestInfo
+{
+    otExtAddress mExtAddress;   ///< IEEE 802.15.4 Extended Address of the requester
+    uint8_t      mVersion : 4;  ///< Thread version.
+    bool         mIsJoiner : 1; ///< Whether is from joiner.
+} otThreadDiscoveryRequestInfo;
+
+/**
+ * This function pointer is called every time an MLE Discovery Request message is received.
+ *
+ * @param[in]  aInfo     A pointer to the Discovery Request info data.
+ * @param[in]  aContext  A pointer to callback application-specific context.
+ *
+ */
+typedef void (*otThreadDiscoveryRequestCallback)(const otThreadDiscoveryRequestInfo *aInfo, void *aContext);
+
+/**
+ * This function sets a callback to receive MLE Discovery Request data.
+ *
+ * @param[in]  aInstance  A pointer to an OpenThread instance.
+ * @param[in]  aCallback  A pointer to a function that is called upon receiving an MLE Discovery Request message.
+ * @param[in]  aContext   A pointer to callback application-specific context.
+ *
+ */
+void otThreadSetDiscoveryRequestCallback(otInstance *                     aInstnace,
+                                         otThreadDiscoveryRequestCallback aCallback,
+                                         void *                           aContext);
 /**
  * @}
  *