accept custom allocator for LoanedMessage. (#2672)

* accept custom allocator for LoanedMessage.

Signed-off-by: Tomoya Fujita <Tomoya.Fujita@sony.com>

* move message allocator using directives to public.

Signed-off-by: Tomoya Fujita <Tomoya.Fujita@sony.com>

---------

Signed-off-by: Tomoya Fujita <Tomoya.Fujita@sony.com>

rclcpp/include/rclcpp/loaned_message.hpp

@@ -31,10 +31,10 @@ namespace rclcpp
 template<typename MessageT, typename AllocatorT = std::allocator<void>>
 class LoanedMessage
 {
+public:
   using MessageAllocatorTraits = rclcpp::allocator::AllocRebind<MessageT, AllocatorT>;
   using MessageAllocator = typename MessageAllocatorTraits::allocator_type;
 
-public:
   /// Constructor of the LoanedMessage class.
   /**
    * The constructor of this class allocates memory for a given message type
@@ -57,7 +57,7 @@ class LoanedMessage
    */
   LoanedMessage(
     const rclcpp::PublisherBase & pub,
-    std::allocator<MessageT> allocator)
+    MessageAllocator allocator)
   : pub_(pub),
     message_(nullptr),
     message_allocator_(std::move(allocator))