[maintenance idea] Benchmark continously via Codspeed

[webknjaz]

So looking at #2082, it occurred to me that it would be nice to have metrics/trends on performance. And I remembered that we use Codspeed for this in @aio-libs. It's a relatively new but rather insightful service. Here's what the web UI looks like: https://codspeed.io/aio-libs/yarl.

The idea is having a bunch of simple tests marked with the respective pytest marker and run them in a dedicated GHA job, eliminating any flakiness factors. The timings are reported to their web service and can then be used in PR checks, on the web UI and their app can leave comments in PRs if some of the benchmarks deviate from the previous runs.