Reexport `EventReceiver` in a generated client crate (#3305)

## Motivation and Context
Adds re-export for `EventReceiver` in a generated client crate

## Description
`EventReceiver` was added in
https://github.com/smithy-lang/smithy-rs/pull/3114

 but the struct itself
could not be referenced as `pub`. Our SDK integration tests did not
break in the said PR because they only called the `next` method on it
and never referred to that struct name.

This PR adds a re-export for `EventReceiver` in case customers need to
refer to the name in their code.

## Testing
Added a unit test and edited a `transcribestreaming` integration test to
verify the re-export.

## 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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choice._

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Co-authored-by: AWS SDK Rust Bot <aws-sdk-rust-primary@amazon.com>