[multicast] Add multicast group management to the softnpu management protocol

This extends the management message protocol with operations for multicast
group lifecycle: create, destroy, add port, remove port, and list. These dispatch
to the corresponding [p4rs](https://github.com/oxidecomputer/p4) Pipeline trait
methods (add_mcast_group, remove_mcast_group, add_mcast_port, etc.) that were added
to support bifurcated multicast replication in the p4rs codegen.

Mutating operations (create, destroy, add, remove) are fire-and-forget, matching the
existing TableAdd/TableRemove pattern. `MulticastGroupList` is the only round-trip
operation, returning a sorted group-to-ports mapping for use by dendrite's
`AsicMulticastOps` queries.

The `MulticastPortAdd` struct carries rid and level1_excl_id for API
parity with dendrite's AsicMulticastOps trait, though softnpu does not
use them. Tofino's PRE handles per-replica identification and
exclusion via these fields. softnpu handles the equivalent via
`McastReplicationTag` in the codegenning. 

This branch depends on the related oxidecomputer/p4#240.

Author: zeeshanlakhani