Implement client side of buxton list-keys command

This will enable clients to query all keys found in a given layer.
The array struct declaration was moved into bt-daemon.h to prevent
any issue in inclusion order (one needs the other)

This commit does not add backend or wire protocol support for
the list-keys command.
Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>