pidl:NDR/Server: Allow to define endpoint server shutdown functions

The next commits will register legacy api_struct when the endpoint server
is initialized. This commit adds a shutdown function which will be used
to unregister the legacy api_struct.

The shutdown function will be also used to replace the rpc_srv_callbacks
struct shutdown member used, for example, by the spoolss service to
cleanup before exiting.

Signed-off-by: Samuel Cabrero <scabrero@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
