Add a middleware vs. orchestrator performance comparison benchmark (#2593)

## Motivation and Context

This PR adds a benchmark suite to the `aws-sdk-s3` test in
`aws/sra-test` so that the performance of client middleware can be
compared against the new orchestrator approach.

The comparison is not yet apples to apples since the orchestrator isn't
complete enough to be comparable, but this is a benchmark that we can
check regularly as they become more comparable.

Initially, the performance was really bad (43x slower than middleware),
but that turned out to be a trivial mistake in orchestrator
implementation that was causing some configuration JSON to get reparsed
for every request. Fixing that makes the orchestrator only about 1.5x
slower, and there is another trivial optimization that can be made to
bring that down more.

The benchmark can be run with the following:
```bash
smithy-rs$ ./gradlew aws:sra-test:assemble
$ cd aws/sra-test/integration-tests/aws-sdk-s3
$ cargo bench
```

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