Add benchmarks

[davidpdrsn]

The recent discussions around the TechEmpower benchmarks showed that, while axum is plenty fast, having some benchmarks in the repo would be useful:

• The purpose of the benchmarks should be to compare axum to itself such that one can easily see if some change improves performance.
• I don't they should do git shenanigans to automatically compare to a previous commit. Just print numbers for the current commit.
• They should not be designed to compare axum to other frameworks. There are other benchmarks that do that.
• They should be easy to run. Ideally just cargo bench if the output is good enough. Some bash script using rewrk is also fine with me.
• They should exercise things most users will use like routing and JSON input/output. Non axum specifics, like databases, aren't relevant imo.
• It's important that the benchmarks don't use unrealistic code. They should use the APIs we expect users to use.
• I don't necessarily think we should run the benchmarks on CI but should at least type check them to prevent bit rot.

I don't intend to chase micro benchmarks but having some convenient scripts to run does have value imo.