Draft: perf: turn off reliable mode for WebRTC DataChannel

Not a networking expert, but from what I understand, the reliability stuff is done on the wrapped protocols, so we can turn the reliability off for the client-to-proxy connection in order to not do it twice. The same could be done for proxy-to-server, but it's WebSocket, there's no unreliable mode there (maybe we could switch to WebRTC there as well *wink*).

Did not test this at all.

TODO:

• Think if it's a good idea.
• Ahh. Apparently a bit of reliability is still required: #33897 (comment 2595413). Can't we do this on a separate reliable channel?
• Also probably need to make the send queue small, see !144 (merged) and snowflake-webext#5 (comment 2851591)
• This a breaking change for the Snowflake the library. Need to make it optional.

Related: #40136 (closed).