[message] track the MLE command type in Message (#10809)

This commit adds a new field, `mMleCommand`, to `Message::Metadata` to
track the MLE command type of the message. Helper methods, such as
`IsMleCommand()`, are added to `Message` to get/set/check the MLE
command type.

This replaces the previous model where `Message::SubType` was used to
track a subset of MLE commands. It helps simplify the code and allow
us to track all MLE commands.

tests/unit/test_message.cpp

@@ -70,7 +70,7 @@ void TestMessage(void)
     message->SetLinkSecurityEnabled(Message::kWithLinkSecurity);
     SuccessOrQuit(message->SetPriority(Message::Priority::kPriorityNet));
     message->SetType(Message::Type::kType6lowpan);
-    message->SetSubType(Message::SubType::kSubTypeMleChildIdRequest);
+    message->SetSubType(Message::SubType::kSubTypeJoinerEntrust);
     message->SetLoopbackToHostAllowed(true);
     message->SetOrigin(Message::kOriginHostUntrusted);
     SuccessOrQuit(message->SetLength(kMaxSize));