Reexport EventReceiver in a generated client crate

[ysaito1001]

Motivation and Context

Adds re-export for EventReceiver in a generated client crate

Description

EventReceiver was added in #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

• 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

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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[rcoh]

LGTM. At some point in the future, we can change this to use automatic re-export

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• Client Test (ignoring whitespace)
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.