Multiplexing, UDP and TUN device support

This update brings multiplexing as well as experimental TUN device support for the client (on Linux) which forwards all traffic (UDP and TCP) over the circuit.

Please note, this is a backwards incompatible release.

Changes

client

• Implemented multiplexed circuit connections via HTTP/2 transport.
• Added experimental TUN device support (Linux).
• Added tun sub-command for TUN device usage.
• Added local H2C listening port for TUN interaction.
• Added SOCKSv5 UDP relay functionality.

relay

• Implemented multiplexed circuit connections via HTTP/2 transport.
• Circuit connections as HTTP/2 streams with single TCP connection.
• Deprecated https:// relay URL scheme.

directory

• Do not restrict relay enrollment URL scheme (no longer applies).