quic: add a config knob for congestion control algorithm

Right now the default is BBR and enabled by hard coding a QUICHE reloadable flag in
#19349 quic: enable BBR congestion control. But we want it to be configurable, especially distinguishable between upstream and downstream.
As discussed with @ggreenway, the preferred default value should be CUBIC, in consideration of https://en.m.wikipedia.org/wiki/TCP_congestion_control#TCP_BBR.