[message] add API functions to enable multicast looping of a message

At the moment, C API does not allow to get/set the configuration flag
which the message to be looped back to the Thread interface. It is
important within the context of sending already prepared IPv6 mutlicast
packets with `otIp6Send()`.

include/openthread/message.h

@@ -211,6 +211,28 @@ bool otMessageIsLoopbackToHostAllowed(const otMessage *aMessage);
  */
 void otMessageSetLoopbackToHostAllowed(otMessage *aMessage, bool aAllowLoopbackToHost);
 
+/**
+ * Indicates whether the given message may be looped back in a case of a multicast destination address.
+ *
+ * If @p aMessage is used along with an `otMessageInfo`, the `mMulticastLoop` field from `otMessageInfo` structure
+ * takes precedence and will be used instead of the the value set on @p aMessage.
+ *
+ * This API is mainly intended for use along with `otIp6Send()` which expects an already prepared IPv6 message.
+ *
+ * @param[in]  aMessage A pointer to the message.
+ *
+ */
+bool otMessageIsMulticastLoopEnabled(otMessage *aMessage);
+
+/**
+ * Controls whether the given message may be looped back in a case of a multicast destination address.
+ *
+ * @param[in]  aMessage  A pointer to the message.
+ * @param[in]  aEnabled  The configuration value.
+ *
+ */
+void otMessageSetMulticastLoopEnabled(otMessage *aMessage, bool aEnabled);
+
 /**
  * Gets the message origin.
  *

src/core/api/message_api.cpp

@@ -62,6 +62,16 @@ void otMessageSetLoopbackToHostAllowed(otMessage *aMessage, bool aAllowLoopbackT
     return AsCoreType(aMessage).SetLoopbackToHostAllowed(aAllowLoopbackToHost);
 }
 
+bool otMessageIsMulticastLoopEnabled(otMessage *aMessage)
+{
+    return AsCoreType(aMessage).GetMulticastLoop();
+}
+
+void otIp6SetMulticastLoopEnabled(otMessage *aMessage, bool aEnabled)
+{
+    AsCoreType(aMessage).SetMulticastLoop(aEnabled);
+}
+
 otMessageOrigin otMessageGetOrigin(const otMessage *aMessage) { return MapEnum(AsCoreType(aMessage).GetOrigin()); }
 
 void otMessageSetOrigin(otMessage *aMessage, otMessageOrigin aOrigin)