[multicast] Narrow underlay scope to ff04::/64 and add ASM source filtering

Re-opening this from #162, as that PR was getting pretty gnarly with a saturation of merge commits. This one deserves a re-review after some added changes to match the Omicron mcast-lifecycle work in oxidecomputer/omicron#9450.

Underlay changes:
- Restrict internal multicast from admin/site/org scoped (ff04, ff05, ff08) to just Omicron's reserved subnet (ff04::/64). These underlay addresses are made unique in Omicron.
- Rename `AdminScopedIpv6` to `UnderlayMulticastIpv6`.
- Simplify P4 to only match ff04::/64.
- Use omicron-common multicast constants for validation.

Source filtering changes:
- Replace IpSrc::Subnet with IpSrc::Any for any-source matching.
- Change IPv6 source filter from exact to LPM match.
- Allow source filters on ASM groups (previously SSM-only).

API changes:
- API v5 adds IpSrc::Any for ASM source filtering.
- API v6 enforces strict underlay subnet validation.

Reviewers: FelixMcFelix, Nieuwejaar, rcgoodfellow

Pull Request: #189

Author: zeeshanlakhani

.cargo/config.toml

@@ -6,3 +6,7 @@ xtask = "run --package xtask --"
 # See https://doc.rust-lang.org/cargo/reference/config.html#target.
 [build]
 rustflags = ["--cfg", "tokio_unstable"]
+
+# Workaround for omicron's bootstore transitive dependency on pq-sys
+[env]
+DEP_PQ_LIBDIRS = "1"