Allow/support arbitrary L4 protocols in guest traffic

[taspelund]

OPTE currently rejects traffic that doesn't match one of 4 permitted L4 protocols: TCP, UDP, ICMP and ICMPv6.

While there are NAT considerations for other L4 protocols (Shared NAT / PAT / NAT Overload / rely on TCP/UDP source/destination ports as part of the NAT entry, so it's only compatible with 1:1 NAT), this is restrictive for users who need to rely on other transport protocols.

e.g.

Keepalived uses VRRP to signal state transitions (must be unicast on our platform), but since this rides inside its own L4 Protocol, those packets get dropped by OPTE.

Simple tunneling mechanisms like GRE or IP-in-IP rely on L4 protocols that are not TCP, UDP or ICMP.

In these situations, the narrow L4 protocol support is a hindrance... where at best the experience is functional but kludgy (like having to setup something like wireguard as transport).