Use codspeed runner for wall time benchmarking (#13586)
We couldn't use the CodSpeed "walltime" runner because it required
administrative permissions on our repositories, but following some
feedback they've adjusted the required permissions so we can give it a
try now.
As a brief background, CodSpeed uses Valgrind for instrumented
benchmarking, emulating the execution for improved stability on GitHub's
runners. This is nice, but means things like allocs and io are not
measured. Now, they support standard wall time benchmarking, using their
own managed runners for stable measurements. Here, we add support for
those while retaining the old workflow — you can toggle between views in
their UI.
