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>