Fix the middleware benchmark and rerun (#2840)

## Motivation and Context
Rerun the middleware benchmark.

Results:
```
    Finished bench [optimized + debuginfo] target(s) in 3.77s
     Running benches/middleware_vs_orchestrator.rs (/Users/rcoh/code/smithy-rs/target/release/deps/middleware_vs_orchestrator-eec2a5ed375836b0)
compare/middleware (HEAD)/S3 ListObjectsV2
                        time:   [27.887 µs 27.956 µs 28.030 µs]
                        change: [+22.004% +22.457% +22.923%] (p = 0.00 < 0.05)
                        Performance has regressed.
Found 6 outliers among 100 measurements (6.00%)
  5 (5.00%) high mild
  1 (1.00%) high severe
compare/middleware (last_release)/S3 ListObjectsV2
                        time:   [22.076 µs 22.122 µs 22.176 µs]
                        change: [-3.1947% -2.7875% -2.4454%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  5 (5.00%) high mild
  2 (2.00%) high severe
compare/orchestrator/S3 ListObjectsV2
                        time:   [27.711 µs 27.764 µs 27.821 µs]
                        change: [-11.548% -11.313% -11.097%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 9 outliers among 100 measurements (9.00%)
  6 (6.00%) high mild
  3 (3.00%) high severe
```

## Description
- The fixup plugin is not required anymore.
- Added a readme with instructions
## Testing
- ran the benchmark


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