Avoid exposing `aws_smithy_http::event_stream::receiver::Receiver` in SDK's public API (#3114)

## Motivation and Context
Implements #3100 

## Description
Currently, we expose `aws_smithy_http::event_stream::Receiver` in
generated SDKs, as shown in the following S3's example (see[ a generated
diff](https://d2luzm2xt3nokh.cloudfront.net/codegen-diff/cc303ab1a07693ab02d5ec4f06101b628d1dbabe/1aa6a8da7d2b7669ba3ab7179a1fd72aadc03162/aws-sdk-ignore-whitespace/index.html)
for
`tmp-codegen-diff/aws-sdk/sdk/s3/src/operation/select_object_content/_select_object_content_output.rs`):
```
pub struct SelectObjectContentOutput {
    <p>The array of results.</p>
    pub payload: ::aws_smithy_http::event_stream::Receiver<
        crate::types::SelectObjectContentEventStream,
        crate::types::error::SelectObjectContentEventStreamError,
    >,
...
```

This PR wraps `Receiver` in a new-type, called `EventReceiver`, which
then supports `pub async fn recv` method whose signature is the same as
`aws_smithy_http::event_stream::Receiver::recv`.

## Testing
Relied on existing tests (e.g. `s3` and `transcribestreaming`
integration tests cover uses cases affected by this change).

## 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

----

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