pf: Add support for endpoint independent NAT bindings for UDP

With Endpoint Independent NAT bindings for UDP flows from a NATed source
address are always mapped to the same ip:port pair on the NAT router.
This allows a client to connect to multiple external servers while
appearing as the same host and enables NAT traversal without requiring
the client to use a middlebox traversal protocol such as STUN or TURN.

Introduce the 'endpoint-independent' option to NAT rules to allow
configuration of endpoint independent without effecting existing
deployments.

This change satisfies REQ 1 and 3 of RFC 4787 also known as 'full cone'
NAT.

Using Endpoint Independent NAT changes NAT exhaustion behaviour it does
not introduce any additional security considerations compared to other
forms of NAT.

PR: 219803
Co-authored-by: Damjan Jovanovic <damjan.jov@gmail.com>
Co-authored-by: Naman Sood <mail@nsood.in>
Reviewed-by: kp
Sponsored-by: Tailscale
Sponsored-by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11137