Bluetooth: host: add dedicated WQ for long-running tasks

Send long-running tasks to a dedicated low-priority workqueue.

This shouldn't increase memory usage since by doing this, we get rid of the
ECC processing thread.

This should fix issues like #43811, since the system workqueue runs at a
cooperative priority, and the new dedicated one runs at a pre-emptible
priority.

Fixes #43811

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>