Add C++ interface for the libbabeltrace2 `bt_message` API

Add C++ interface for the libbabeltrace2 `bt_message` API

This patch adds C++ wrappers for Babeltrace 2 message objects.

The class hierarchy is:

    Message
      StreamBeginningMessage
      StreamEndMessage
      PacketBeginningMessage
      PacketEndMessage
      EventMessage
      DiscardedEventsMessage
      DiscardedPacketsMessage
      MessageIteratorInactivityMessage
    ConstMessage
      ConstStreamBeginningMessage
      ConstStreamEndMessage
      ConstPacketBeginningMessage
      ConstPacketEndMessage
      ConstEventMessage
      ConstDiscardedEventsMessage
      ConstDiscardedPacketsMessage
      ConstMessageIteratorInactivityMessage

Implicitly convert from a mutable field class to a constant field class
with converting constructors and assignment operators.

Those new template classes follow the approach of other wrappers in
`src/cpp-common/bt2`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: If01ac4b2bbf8eb1ea400f8e5e7c2003b4844b80f
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4735