Add codspeed benchmarking in a dedicated CI job

[sirosen]

Also add a new tox config to support running codspeed locally and in CI.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

CodSpeed was successfully installed, but no benchmarks were detected in the uploaded report.

Please make sure your benchmarks are correctly instrumented by following the benchmarks creation guide.

If you run into issues, reach out at support@codspeed.io or on our Discord.

[sirosen]

I think this will not run cleanly until merged, because codspeed is looking for baseline data from main. I can do some hacky workaround stuff, but will first raise the possibility, with the team, of merging as-is so that I can simply trigger on main.