Fix SigV4 Initial Message support

[rcoh]

Motivation and Context

#4397

The SigV4 support for event streams did not unframe the message before trying to read the initial event. This mean that if an initial event was sent, it ended up going to the unmarshaller where it subsequently caused an error.

Description

The previous attempt at this wrapped the Unmarshaller with a SigV4Unmarshaller. This didn't quite work in practice.

Previously, SigV4 support worked by transforming the receiver type:

Receiver<T, E> → Receiver<SignedEvent<T>, SignedEventError<E>>

This approach had a fundamental flaw: the receiver's internal buffering logic didn't understand SigV4 envelopes. When it buffered a message, it would buffer the unwrapped inner message, losing the signature.

The new design introduces SigV4Receiver<T, E> as a wrapper around Receiver<SignedEvent<T>, SignedEventError<E>>.

This allows us to both capture the initial message signature (which will be required to actually verify it) as well as providing a clean place to handle the details.

This also meant that I could back out all the previous customization changes, so those are now deleted.

Testing

The tests were cleaned up. Most were moved to a rstest to handle all the individual test behaviors.

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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