feat: Port Control Protocol (PCP) / rfc6887 support

[dozyio]

Adds PCP support - https://www.rfc-editor.org/rfc/rfc6887

Supports

• Private IPv4 -> Private IPv4 NAT -> Pubic IPv4
• IPv6 GUA -> IPv6 Firewall -> IPv6 GUA

Unsupported

• Rapid recovery via mutlicast
• IPv6 privacy address rotation
• PEER opcode
• Multiple gateways for same IP version

Running tests

IPv6 - GATEWAY needs to be the routers IPv6 GUA LAN address so that the host uses the correct source address

GATEWAY=xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx npm run test -- --grep "pcp"

IPv4 - GATEWAY needs to be the routers private NAT interface

GATEWAY=x.x.x.x npm run test -- --grep "pcp"

Having a bit of trouble updating the readme via npm run docs - it deletes the About section... any ideas?

[dozyio]

@achingbrain this should be ok to review now 🙏

[dozyio]

@achingbrain any chance you can take a look at this or I should I put it in its own package? :)