Onionperf ability to configure guard topology and proximity

In tpo/core/tor#40157 (closed) we've been doing experiments with various guard topologies. In particular we do experiments with guards that are close to the onionperf box, and guards that are far from the onionperf box, or with 2 guards where one guard is close and the other is far. We also do experiments with a single guard and two guards.

Right now I'm doing this by specifying a different config file for each of those scenarios. However if we want to run these experiments consistently in the future we might want to automate this in onionperf.

\cc @acute @karsten @mikeperry