Dual-Stack Endpoints and IPv6 ICE Candidates

[stefankiesz]

Issue #, if available:
N/A

What was changed?

• Added support for dual-stack KVS signaling channel endpoints.
• Added support for dual-stack KVS STUN servers and IPv6 srflx candidates.
• Added support for dual-stack KVS TURN servers and IPv6 relay candidates.
  • Dual-stack TURN servers
• Added an environment variable to switch between dual-stack and legacy (default) modes.

Why was it changed?

• To prepare for dual-stack KVS service support.

How was it changed?

• ICE server structs now maintain two IP addresses per server: IPv4 and IPv6.
  • If the ICE server is not dual-stack, then only the IPv4 address will be set.
• Dual-stack peers using dual-stack STUN servers gather both IPv4 and IPv6 srflx candidates, allowing for cross-network-type connections between dual-stack and IPv4-only peers.
• 2 turn connections (IPv4 and IPv6) will be made if using dual-stack TURN service; the IPv6 allocation requests will include the REQUESTED-ADDRESS-FAMILY attribute.

What testing was done for the changes?

• CI to pass.
• Unit tests with dual-stack server resolution to be added once service support is enabled.

This PR includes changes from #2115.

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