This commit introduces IGMPv3 detection and parsing.

When bpf_lxc recognizes IGMP messages egressing the Pod we attempt to
parse them.

The parsing logic is as follows:
1. Determine if traffic is IGMP
2. Determine the IGMP message type
3. If the type is not a membership report simply drop it (for now)
4. Parse each group record in the membership report
5. For any group records which indicate a join add a subscriber to the
   multicast subscriber map, if it exists.

Signed-off-by: Louis DeLosSantos <[email protected]>

This commit adds parsing of IGMPv2 messages in a similar fashion as
IGMPv3 messages.

Signed-off-by: Louis DeLosSantos <[email protected]>

This commit implements replication and delivery of multicast packets.

This commit also enables the Cilium datapath to access both `bpf_clone_redirect`
and `bpf_map_for_each_elem` helpers.

The datapath flow is illustrated below:

┌──────────────────────────────────────────┐
│                                          │
│  Sender                                  │
│  ┌──────┐     ┌─────────┐                │
│  │ pod  ├─────► bpf_lxc │                │
│  └──────┘     └────┬────┘                │
│  Local Receivers   │  eBPF Replication   │
│  ┌──────┐ ┌──────┐ │  and Redirection    │
│  │ pod  ◄─┤ veth ◄─┤(cil_from_container) │
│  └──────┘ └──────┘ │ ┌───────┐           │
│                    ├─► vxlan │           │
│  ┌──────┐ ┌──────┐ │ └───┬───┘           │
│  │ pod  ◄─┤ veth ◄─┘     │               │
│  └──────┘ └──────┘  ┌────┘               │
│                     │                    │
└─────────────────────┼────────────────────┘
                      │
┌─────────────────────┼────────────────────┐
│                     │                    │
│                 ┌───▼───┐                │
│                 │ vxlan │                │
│                 └───┬───┘                │
│   Remote Receivers  │  eBPF Replication  │
│   ┌──────┐ ┌──────┐ │  and Redirection   │
│   │ pod  ◄─┤ veth ◄─┤  (from_overlay)    │
│   └──────┘ └──────┘ │                    │
│                     │                    │
│   ┌──────┐ ┌──────┐ │                    │
│   │ pod  ◄─┤ veth ◄─┘                    │
│   └──────┘ └──────┘                      │
│                                          │
└──────────────────────────────────────────┘

A multicast sender sends a multicast packet.

The sender's bpf_lxc program does a lookup in the multicast group map to
discover who has subscribed to the group.

The program then clones and redirects the packets to the subscriber's
ingress device on the host namespace.

If the subscriber is remote the packet is cloned and redirected to a
vxlan device for encapsulation.

Once the host stack forwards the vxlan encap'd packet to the receiving
vxlan device on the remote host a similar "clone and redirect" process
is performed once the vxlan driver decaps the packet.

Signed-off-by: Louis DeLosSantos <[email protected]>