build(vcpkg): Build GameNetworkingSockets from our fork with the ICE fixes (a3)

[tintinhamans]

Builds GNS from our fork until the ICE fixes are merged upstream.

Stacked on #604, so review only the last commit.

• re-added the ./ports overlay with the upstream 1.6.0 port, pointed at the fork
• the fork is upstream master plus:
  • ICE: Dedupe servers and track the server index on STUN and TURN requests ValveSoftware/GameNetworkingSockets#438: STUN/TURN failover tracks the server index, so duplicate addresses no longer loop forever
  • Native ICE resolves STUN/TURN hostnames under the global lock ValveSoftware/GameNetworkingSockets#440: STUN/TURN hostnames are resolved off the global lock, so slow DNS no longer stalls every connection
  • duplicate STUN/TURN addresses are skipped, so they don't cost a timeout each
  • WebRTC ICE keeps an idle thread per STUN/TURN hostname lookup ValveSoftware/GameNetworkingSockets#439 (WebRTC resolver threads) is included but unused with native ICE
• once those are merged and in the vcpkg port, the overlay can be dropped again