Bluetooth: Host make the long workqueue stack size configurable

Add a prompt to the long workqueue stack size, so that it can be easily
fine-tuned by the application. The exact usage will depend on many factors,
such as the chosen crypto backend and target architecture, so it's not
possible to have "one size fits all" solutions based on the default values.

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