Merge a turbotunnel branch

Snowflake turbo tunnel features have now been through a test deployment (#33336 (closed)) and a few iterations of Tor Browser packages. There haven't been as many test reports as I'd like, but what testing there has been has been mostly positive. Turbo tunnel–like features are a dependency of some of the tasks for a stable release of Snowflake (#19001 (closed)). So we should merge it.

Some sub-tasks:

• Decide between the KCP and QUIC branch.
• Test without LearnCircuitBuildTimeout 0 and find another workaround, if necessary. See #33336 (comment 2595075).
• Rebase and clean history of the chosen branch.
• Redeploy bridge from master.

Summary of turbo tunnel development history till now:

• Turbo Tunnel in Snowflake
• Second draft of Turbo Tunnel Snowflake packages
• Third draft of Turbo Tunnel Snowflake packages
• Trial deployment of Snowflake with Turbo Tunnel
• Support multiple simultaneous SOCKS connections

One bug that may or not be snowflake's fault:

• "Pluggable Transport process terminated" but Tor keeps on going (and of course doesn't work)