Bluetooth: host: l2cap: add alloc_seg callback

This callback allows use-cases where the SDU is much larger than the
l2cap MPS. The stack will then try to allocate using this callback if
specified, and fall-back on using the buffer's pool (previous
behavior).

This way one can define two buffer pools, one with a very large buffer
size, and one with a buffer size >= MPS, and the stack will allocate
from that instead.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
(cherry picked from commit 77e1a9dc)