Make it possible to customize presigned requests (#3385)

## Motivation and Context
This enables the following code:
```rust
    let req = client
        .get_object()
        .bucket("foo")
        .key("bar")
        .customize()
        .mutate_request(|req| {
            req.set_uri(req.uri().to_string() + "&a=b")
                .expect("failed to update URI")
        })
        .presigned(static_ps_config)
        .await
```
Previously, it wasn't possible to presign a request once customization
had begun.


## Description
- Follow the same pattern as send by introducing an additional trait,
CustomizeablePresign
- change InvokeWithStopPoint to actually _return_ the error if there is
one.

## Testing
New integration test

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

----

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