Carry IPv4 routes over IPv6 sessions with IPv6 next hops (RFC 8950)

For a link that has only IPv6 addresses.

- The Extended Next Hop capability is offered on every session over IPv6
  whose families include IPv4 unicast, and used if the peer offers it too.
  Nothing to configure.
- On such a session our IPv4 routes go out in MP_REACH_NLRI with the
  session's IPv6 address as next hop, and are withdrawn in
  MP_UNREACH_NLRI. Without the agreement they are not sent, as before.
- IPv4 routes are read with an IPv6 next hop, global or global with
  link-local.
- The kernel installer resolves an IPv4 route's IPv6 next hop like any
  other and installs it: by way of an IPv6 nexthop object, or with RTA_VIA
  where routes carry their own gateway.

Tested against a real kernel in both modes, with a link-local next hop
too, and in the lab against FRR and BIRD: the new `v6only` phase takes
the IPv4 addresses off the exchange LAN, runs one IPv6 session with each
router for both families, and pings IPv4 across.

Sessions between link-local addresses are not supported yet.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/server/src/session.rs

@@ -125,6 +125,9 @@ pub struct SessionHandle {
     pub role: Option<PeerRole>,
     /// Whether the peer said so itself in its OPEN (RFC 9234).
     pub role_confirmed: bool,
+    /// Whether IPv4 routes go with IPv6 next hops on this session
+    /// (RFC 8950).
+    pub extended_next_hop: bool,
     /// Whether the peer negotiated RFC 6793 four-octet ASNs. Determines how
     /// AS_PATH is encoded on the wire for this session.
     pub four_octet_asn: bool,
@@ -170,6 +173,7 @@ impl SessionHandle {
             ibgp: negotiated.ibgp,
             role: None,
             role_confirmed: false,
+            extended_next_hop: negotiated.extended_next_hop,
             four_octet_asn: negotiated.four_octet_asn,
             families: negotiated.families.clone(),
             add_path_receive: negotiated.add_path_receive.clone(),