[zeeshanlakhani]

This adds a swadm multicast subcommand (aliased as mcast as well) for inspecting, creating, and deleting the multicast groups programmed on the switch:

• multicast list [-t <tag>] [-k external|underlay] renders all groups as a
  table view, filtered by tag, by replication kind, or both.
• multicast get <group-ip> shows one group's full configuration.
• multicast add <ext-ip> -i <underlay> -m <mac> -v <vni> [--member port/link:dir]... [--vlan <id>] [-s <src>]... [-t <tag>] creates an external group and the underlay group that it maps to.
• multicast del <group-ip> [-t <tag>] [--underlay-only] deletes the pair, or just an unreferenced underlay group with --underlay-only.

The kind filter matches on the response type: external means an external multicast group with a NAT target and no (underlying) direction membership; underlay groups reside in the reserved ff04::/64 subnet and replicate to the member ports (to guest subscribers).

add/del subcommands work together as a pair (of groups) by default: add creates the underlay group, then the external group that maps to it (1:1, rolling the underlay back out if the external create fails), and del checks both (shared) tags before removing either group. The -t flag defaults to the client tag on both subcommands, similar to arp add. del --underlay-only removes unreferenced underlay groups, including any left behind after a failed pair deletion.

Includes:

• multicast get runs through TabWriter, like list; list headers use Title Casing, like nat/attached.
• add flags match what nat add has, i.e. -i underlay, -m mac, -v vni, and --member is long-only.
• clap parses right into the new types from [dpd,dpd-api,dpd-types] make external multicast constraints self-enforcing #396 (type-validation), with bad addresses failing at arg parse-time.
• tests moved onto the common expectorate setup from Regression tests for swadm #350 (per Cory's review), seeded through swadm itself, which removes the tests' need for tokio and the dropshot client.

[cfzimmerman]

Wow this totally rocks. Tysm for updates after the last review. Awesome functionality and well tested.