Bluetooth: Add dedicated pool for HCI_Num_Completed_Packets HCI event

This event is a priority one, so it's not safe to have it use the RX
buffer pool which may be depleted due to non-priority events (e.g.
advertising events). Since the event is consumed synchronously it's
safe to have a single-buffer pool for it. Also introduce a new
bt_buf_get_evt() API for HCI drivers to simplify the driver-side code,
this effectively also deprecates bt_buf_get_cmd_complete() which now
has no in-tree HCI driver users anymore.

Fixes #16864

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>