Expose bbr pacing_rate to users (incl. ffi)

Hello,

I'm using the library for a prototype but would like my application to limit how much data it feeds into the library for sending, in such a way that congestion (RTT increase, loss) is avoided, without incurring buffering into quiche's send buffer.

To do so I would need to be able to query the current pacing rate from the congestion control, but that's not currently exposed. I have a version of this change locally to unblock myself but proper support / consideration of any congestion control algorithm would of course be preferred. My version is a simple forwarding of the bbr2 state's pacing rate through path stats (and "null" if not using bbr).

Thanks!

Review the path stats and bbr2 congestion-control state described in the issue, then inspect the existing FFI surface. Done means applications can query the current pacing rate through path stats, with defined behavior when bbr is not used, and the support is available through FFI.

Written by the indexing model from the issue text.