Adds experimental mocks package. (#3308)

Publishing is disabled for this package currently.

## Motivation and Context
Testing is a huge pain. Currently. This method isn't ideal and can lack
accuracy in some cases but in quick and useful for many applications.

## Description
New mock package that allows code like this:
```rust
    let s3_404 = mock!(Client::get_object)
        .match_requests(|inp| {
            inp.bucket() == Some("test-bucket") && inp.key() != Some("correct-key")
        })
        .then_http_response(|| {
            http::Response::builder()
                .status(400)
                .body(SdkBody::from(S3_NO_SUCH_KEY))
                .unwrap()
                .try_into()
                .unwrap()
        });

    let s3_real_object = mock!(Client::get_object)
        .match_requests(|inp| {
            inp.bucket() == Some("test-bucket") && inp.key() == Some("correct-key")
        })
        .then_output(|| {
            GetObjectOutput::builder()
                .body(ByteStream::from_static(b"test-test-test"))
                .build()
        });

    let get_object_mocks = MockResponseInterceptor::new()
        .with_rule(&s3_404)
        .with_rule(&s3_real_object);

    let s3 = aws_sdk_s3::Client::from_conf(
        Config::builder()
            .with_test_defaults()
            .region(Region::new("us-east-1"))
            .interceptor(get_object_mocks)
            .build(),
    );
```

There is no binary level mocking and this is completely typesafe. IDE
inference works properly as well. The crimes that were committed were
small and self-contained.

## Testing
There is a unit test and it works.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._