Bluetooth: Add support for Get Advertising Size Information command

The Get Advertising Size Information command allows to retrieve size
information for advertising data and scan response data fields depending
on the selected flags. This is useful if applications want to know the
available size ahead of time.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>