Enable IPv6 address discovery

[Genteure]

• Enabled IPv6 address discovery, this allows clients to establish connection directly via IPv6 when available.
• Added a new config option DisableIPv6Discovery to revert the behavior back to IPv4 only. Default value is false, so existing config will default to using IPv6 when possible without user manually changing the config file.

Tested between my Windows 10 PC and a Raspberry Pi, using the hosted management and signal server (https://app.netbird.io/)

Closes #577

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Some notes:

Link-local IPv6 addresses (e.g. fe80:*) are filtered out, unlike IPv4 address (e.g. 10.0.4.3 is not filtered out).
I don't have a Unique local address so I don't know if that's also being filtered out.

ice.Candidate with IPv6 host are formatted similar to fe80::1:51820, this only affects the log output, might be a good idea to submit a fix upstream later.

Filtered log from testing:

Public keys are replaced with <WG_PUBLIC_KEY_1> <WG_PUBLIC_KEY_2>
IPv4 Addresses are replaced with 999.999.999.999
IPv6 Addresses are masked with xxxx

conn.go:559 OnRemoteOffer from peer <WG_PUBLIC_KEY_1> on status Disconnected
conn.go:565 OnRemoteOffer skipping message from peer <WG_PUBLIC_KEY_1> on status Disconnected because is not ready
conn.go:190 trying to connect to peer <WG_PUBLIC_KEY_1>
conn.go:221 connection offer sent to peer <WG_PUBLIC_KEY_1>, waiting for the confirmation
conn.go:574 OnRemoteAnswer from peer <WG_PUBLIC_KEY_1> on status Disconnected
conn.go:242 received connection confirmation from peer <WG_PUBLIC_KEY_1> running version development and with remote WireGuard listen port 51820
conn.go:476 peer <WG_PUBLIC_KEY_1> ICE ConnectionState has changed to Checking
conn.go:121 ignoring interface wt0 - it is not allowed
conn.go:459 discovered local candidate udp4 host 192.168.1.8:42332
conn.go:459 discovered local candidate udp6 host xxxx:xxx:xxxx:xxxx::a000:42332
conn.go:459 discovered local candidate udp6 host xxxx:xxx:xxxx:xxxx:xxxx:xxxx:xxxx:c1ae:42332
conn.go:121 ignoring interface wt0 - it is not allowed
conn.go:459 discovered local candidate udp4 srflx 999.999.999.999:27835 related :::57778
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp6 host xxxx:xxx:xxxx:xxxx::a001:51423
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp6 host xxxx:xxx:xxxx:xxxx:xxxx:xxxx:xxxx:ffe0:51423
noproxy.go:37 using NoProxy while connecting to peer <WG_PUBLIC_KEY_1>
conn.go:476 peer <WG_PUBLIC_KEY_1> ICE ConnectionState has changed to Connected
conn.go:470 selected candidate pair [local <-> remote] -> [udp6 host xxxx:xxx:xxxx:xxxx::a000:42332 <-> udp6 host xxxx:xxx:xxxx:xxxx::a001:51423], peer <WG_PUBLIC_KEY_1>
configuration.go:94 updating interface wt0 peer <WG_PUBLIC_KEY_1>: endpoint [xxxx:xxx:xxxx:xxxx::a001]:51820
configuration.go:35 got Wireguard device wt0
conn.go:295 directly connected to peer <WG_PUBLIC_KEY_1> [laddr <-> raddr] [xxxx:xxx:xxxx:xxxx::a000:51820 <-> xxxx:xxx:xxxx:xxxx::a001:51820]
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 host 172.28.64.1:51423
conn.go:459 discovered local candidate udp4 relay 54.255.13.85:46396 related 0.0.0.0:50789
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 host 192.168.1.6:51423
conn.go:459 discovered local candidate udp4 relay 54.255.13.85:40497 related 192.168.1.8:60472
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 srflx 999.999.999.999:27806 related :::51424
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 relay 18.198.13.240:40266 related 0.0.0.0:59880
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 relay 18.198.13.240:34423 related 192.168.1.6:63150
conn.go:459 discovered local candidate udp4 relay 52.16.140.95:64436 related 192.168.1.8:37008
conn.go:588 OnRemoteCandidate from peer <WG_PUBLIC_KEY_1> -> udp4 relay 52.16.140.95:30546 related 192.168.1.6:63149

root@raspberrypi:~# wg
interface: wt0
  public key: <WG_PUBLIC_KEY_2>
  private key: (hidden)
  listening port: 51820

peer: <WG_PUBLIC_KEY_1>
  endpoint: [xxxx:xxx:xxxx:xxxx::a001]:51820
  allowed ips: 100.999.999.999/32
  latest handshake: 2 minutes, 49 seconds ago
  transfer: 2.47 KiB received, 1.44 KiB sent
  persistent keepalive: every 25 seconds

[mlsmaycon]

@Genteure, my apologies; I broke your PR for a moment there using Github's conflict resolution in the browser. I should've asked you to update it on your end.

[mlsmaycon]

Awesome, great stuff @Genteure