Add initial benchmarks

[davidpdrsn]

This adds a basic setup for writing benchmarks and adds a couple of benchmarks.

The output looks like this

❯ cargo bench -p axum -- send-json
   Compiling axum v0.5.10
    Finished bench [optimized] target(s) in 0.68s
     Running unittests src/lib.rs (target/release/deps/axum-732f709dd139ce93)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 203 filtered out; finished in 0.00s

     Running benches/benches.rs (target/release/deps/benches-ddcccec176823e7f)
Running "send-json" benchmark
  Beginning round 1...
  Benchmarking 10 connections @ http://0.0.0.0:58714 for 10 second(s)
    Latencies:
      Avg      Stdev    Min      Max
      0.06ms   0.03ms   0.02ms   4.06ms
    Requests:
      Total: 1689726 Req/Sec: 168960.39
    Transfer:
      Total: 228.83 MB Transfer Rate: 22.88 MB/Sec

It basically runs a default hyper server and shells out to rewrk which does all the heavy lifting.

I went with rewrk rather than the default cargo bench tooling (or criterion) because I'm mainly interested in seeing requests per second and not how long a single request takes. I would also like to avoid configuring and running our own client. This seems fine to be me but let me know what you think.

Fixes #1199

[jplatte]

Overall looks very decent :)

[programatik29]

I observed rewrk doesn't exit with error code on error.
Errors can be detected by checking stderr.

[davidpdrsn]

I observed rewrk doesn't exit with error code on error.

Errors can be detected by checking stderr.

We should fix that in rewrk. Checking stderr is tedious 😅