Sign event stream initial messages with signer

[rcoh]

Motivation and Context

#4429

Description

Event stream operations with initial-request messages send that message unsigned, whose root cause is described in the issue above.

To resolve it, instead of pre-serializing the initial message to bytes, this PR

1. creates a wrapper marshaller that can handle both regular events and initial messages
2. creates a stream out of the initial message
3. chains that stream with the event stream
4. lets the entire combined stream flow through MessageStreamAdapter for signing

Testing

Added client codegen test for signing initial message and event.

Checklist

• For changes to the smithy-rs codegen or runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "client," "server," or both in the applies_to key.
• For changes to the AWS SDK, generated SDK code, or SDK runtime crates, I have created a changelog entry Markdown file in the .changelog directory, specifying "aws-sdk-rust" in the applies_to key.

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[github-actions]

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

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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.