[message] simplify method for allocating new message from pool (#7210)

This commit removes different `MessagePool` methods that can be used
to allocate a new message  and combines them into one `Allocate()`
method which uses `Message::Settings`. The `Message::Settings` is
also updated to provide new constructor initializing it with a given
message priority only.

tests/unit/test_heap_string.cpp

@@ -215,7 +215,7 @@ void TestHeapData(void)
     VerifyOrQuit(instance != nullptr, "Null OpenThread instance");
 
     messagePool = &instance->Get<MessagePool>();
-    VerifyOrQuit((message = messagePool->New(Message::kTypeIp6, 0)) != nullptr, "Message::New failed");
+    VerifyOrQuit((message = messagePool->Allocate(Message::kTypeIp6)) != nullptr);
 
     message->SetOffset(0);