See proposal 289 section 4.3 for more details.

It describes the flow control protocol at the circuit and stream level. If
there is no FlowCtrl protocol version, tor supports the unauthenticated flow
control features from its supported Relay protocols.

At this commit, relay will start advertising FlowCtrl=1 meaning they support
authenticated SENDMEs v1.

Closes #30363

Signed-off-by: David Goulet <dgoulet@torproject.org>