sched: Struct definition and parsing of dualpi2 qdisc

sched: Struct definition and parsing of dualpi2 qdisc

DualPI2 is the reference implementation of IETF RFC9332 DualQ Coupled
AQM (https://datatracker.ietf.org/doc/html/rfc9332) providing two
queues called low latency (L-queue) and classic (C-queue). By default,
it enqueues non-ECN and ECT(0) packets into the C-queue and ECT(1) and
CE packets into the low latency queue (L-queue), as per IETF RFC9332 spec.

This patch defines the dualpi2 Qdisc structure and parsing, and the
following two patches include dumping and enqueue/dequeue for the DualPI2.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Link: https://patch.msgid.link/20250722095915.24485-2-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>