[ip6] log encapsulated IPv6 messages (#7178)

When an encapsulated IPv6 header is found, log the message.

src/core/net/ip6.cpp

@@ -1213,6 +1213,7 @@ start:
         {
             // Remove encapsulating header and start over.
             aMessage.RemoveHeader(aMessage.GetOffset());
+            Get<MeshForwarder>().LogMessage(MeshForwarder::kMessageReceive, aMessage, nullptr, kErrorNone);
             goto start;
         }
 

src/core/thread/mesh_forwarder.hpp

@@ -157,6 +157,7 @@ class MeshForwarder : public InstanceLocator, private NonCopyable
     friend class Instance;
     friend class DataPollSender;
     friend class IndirectSender;
+    friend class Ip6::Ip6;
     friend class Mle::DiscoverScanner;
     friend class TimeTicker;
     friend class Utils::HistoryTracker;