Introduce max_outgoing_bytes_per_second option

[aochagavia]

cc @Ralith

This is something we discussed a while ago. It came up in the context of deep space QUIC networking, which requires being able to rate-limit outgoing traffic (traditional congestion control algorithms don't work when an ACK takes minutes to arrive 😅).

[marcblanchet]

fyi. requirement discussed in draft-many-tiptop-quic-profile

[aochagavia]

Looking at the code, it seems like it could also live inside a congestion controller because all we do is manipulate the congestion window. In all honesty, that approach seems cleaner to me than what I've implemented in this PR. What do you think? The only caveat I can think of right now is that the initial congestion window needs to be calculated based on the initial RTT (which is configurable), and the congestion controller doesn't have access to that information.

[Ralith]

I like that, as written, this feature will Just Work with any congestion controller. I'm not absolutely certain how broadly useful it is, but there's definitely a number of internet applications where a manual total bandwidth limit is handy.

[aochagavia]

In that case, let's keep things as-is :)