tipc: add new AEAD key structure for user API

The new structure 'tipc_aead_key' is added to the 'tipc.h' for user to
be able to transfer a key to TIPC in kernel. Netlink will be used for
this purpose in the later commits.

Acked-by: Ying Xue <ying.xue@windreiver.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Tuong Lien <tuong.t.lien@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>