[api] add API to find source address (#3273)

include/openthread/ip6.h

@@ -537,6 +537,17 @@ OTAPI uint8_t OTCALL otIp6PrefixMatch(const otIp6Address *aFirst, const otIp6Add
  */
 bool otIp6IsAddressUnspecified(const otIp6Address *aAddress);
 
+/**
+ * This function perform OpenThread source address selection.
+ *
+ * @param[inout]  aMessageInfo  A pointer to the message information.
+ *
+ * @retval  OT_ERROR_NONE       Found a source address and is filled into mSockAddr of @p aMessageInfo.
+ * @retval  OT_ERROR_NOT_FOUND  No source address was found and @p aMessageInfo is unchanged.
+ *
+ */
+otError otIp6SelectSourceAddress(otInstance *aInstance, otMessageInfo *aMessageInfo);
+
 /**
  * @}
  *