Add IPv6 protocol handlers and integration tests for mvirt-ebpf

Protocol handlers:
- Router Advertisement with M+O flags for DHCPv6 configuration
- DHCPv6 SOLICIT/REQUEST handling with DNS server support
- ICMPv6 Echo Request/Reply for gateway ping6
- Neighbor Solicitation/Advertisement (already existed)

Integration tests (9 new tests):
- RS → RA with M+O flags verification
- RS → RA without O flag when no DNS configured
- DHCPv6 SOLICIT → ADVERTISE with DNS servers
- DHCPv6 REQUEST → REPLY with assigned address
- Full DHCPv6 handshake (RS→RA→SOLICIT→ADVERTISE→REQUEST→REPLY)
- NS → NA for gateway address
- NS for non-gateway returns no response
- ping6 to gateway returns Echo Reply
- ping6 to non-gateway returns no response

Author: MalteJ

mvirt-ebpf/Cargo.toml

@@ -96,3 +96,7 @@ required-features = ["test-util"]
 [[test]]
 name = "service_test"
 required-features = ["test-util"]
+
+[[test]]
+name = "ipv6_test"
+required-features = ["test-util"]