Add support for UDP proxying

[VastBlast]

Reopening #154 as it was auto closed after restructuring my fork.

This PR adds support for a proxying a UDP server through WireGuard. It can listen on IPv4 while tunneling to an IPv6 address or vice versa.

When sending, it goes through BindAddress -> Wireguard -> Target and the opposite when receiving.

For example, if you wanted to proxy Cloudflare's DNS server, an example config would be:

[Interface]
...

[Peer]
...

[UDPProxyTunnel]
BindAddress = 127.0.0.1:53

#Target = [2606:4700:4700::1111]:53
Target = 1.1.1.1:53

# If its set to 0, it will never timeout
InactivityTimeout = 30

[shihiro09]

Love the DNS use-case. cc @whyvl

Just a thought: would this enable nested wireguard tunnels with two wireproxy daemons? e.g.

outer.conf:

[Peer]
Endpoint = [outer-tunnel]:51280
PublicKey = [outer-key]

[UDPProxyTunnel]
BindAddress = 127.0.0.1:1234
Target = [inner-tunnel]:51280

inner.conf:

[Peer]
Endpoint = 127.0.0.1:1234 # inner proxy
PublicKey = [inner-key]

[VastBlast]

Love the DNS use-case. cc @whyvl

Just a thought: would this enable nested wireguard tunnels with two wireproxy daemons? e.g.

outer.conf:

[Peer]
Endpoint = [outer-tunnel]:51280
PublicKey = [outer-key]

[UDPProxyTunnel]
BindAddress = 127.0.0.1:1234
Target = [inner-tunnel]:51280

inner.conf:

[Peer]
Endpoint = 127.0.0.1:1234 # inner proxy
PublicKey = [inner-key]

Yes, it would. Might need more testing, but it should.