Add two public entry points needed by the ALICE use case where shmem
messages are allocated via a transport but never sent — their metadata
is instead serialised into Arrow tables and delivered over a separate
channel, allowing consumer devices to resolve the payload pointer
without taking ownership.
shmem::Message::GetMeta() returns the MetaHeader of the message,
mirroring the existing positional-init pattern already used in Socket.h.
shmem::GetDataAddressFromHandle(TransportFactory&, const MetaHeader&)
is a free function declared in Common.h and defined in Manager.cxx.
Keeping it out of the TransportFactory class body means callers only
need to include Common.h (available transitively via Message.h) and do
not drag in Socket.h or zmq.h. The implementation handles both managed
segments and unmanaged regions, and throws SharedMemoryError with a
typed message on a bad segment or region id. TransportFactory also
gains a same-named member for callers that already have the concrete
type. Lifetime of the returned pointer is the caller's responsibility;
the cache device is expected to hold the messages alive.
A SideChannel test covers the GetMeta/GetDataAddressFromHandle
round-trip for both standard and expanded-metadata configurations.
Callbacks are called when the data buffer of the message assiciated
with the corresponding region is no longer needed by the transport.
Example in examples/advanced/Region/
