if_bridge: add multicast database (MDB) foundation

[namkhanh052005_gmail.com]

Summary

Adds the multicast database (MDB) data structures to if_bridge(4):
struct bridge_mdb_entry and struct bridge_mdb_member, keyed by
(VLAN, multicast group) rather than by the mapped Ethernet MAC alone, so
distinct IP groups sharing an L2 destination are not conflated. Includes
the aging callout scaffolding and the member-detach cleanup path.

No learning or forwarding behavior yet; this patch establishes the data
model only. Also adds a VNET topology helper,
bridge_mdb_vnet_topology_create, reused by every subsequent patch's tests.

Depends on D58729

Test Plan

mdb_vnet_topology ATF test exercises the topology helper. No forwarding
behavior changes at this stage, so there is no forwarding-path test.
Built and booted on both bases; ATF passes.