Bluetooth: Host: Add L2CAP `seg_recv` API

This is an alternative API for the L2CAP receive functionality. It
allows an application the receive L2CAP segments directly and manage
credits explictly. The API is guarded by an experimental kconfig option.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/57485



Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>