Allow public access to raw message object
Allow public access to raw message object

There are certain cases where one would like to access the raw message
object under certain contexts.  In general, these are contexts where
you need runtime access to the message type information, rather than
compile time template typing that the existing message object gives
you.  It is still desirable to use the message object itself for
exception safety and RAII usage semantics.

Specifically, this came up when attempting to replicate the actions
interface in phosphor-rest, where types and conversions are determined
at runtime via introspect, and the type hierarchies are not known at
compile time.

Change-Id: I1e93af30992ffedbf470c1024eefe10cc5e21031
Signed-off-by: Ed Tanous <8bf3d0085830172833e6f5c916cff518b50f87b7@intel.com>

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus