Add full support for AWS chunked content encoding

[ysaito1001]

Motivation and Context

#1798
#4188

Description

This PR adds full support for AWS chunked content encoding, addressing limitations documented in these comments.

Customer impact

This PR affects S3's PutObject and UploadPart operations with streaming payloads, changing their default behavior:

Before: Streaming payloads were sent as a single chunk.

After: Streaming payloads are split into 64 KiB (the last chunk may be smaller) and sent in multiple chunks.

Customers who prefer the previous single-chunk behavior can disable chunking via the service config:

let config = aws_sdk_s3::Config::builder()
    .aws_chunked_encoding_chunk_size(None) // Use entire content as one chunk
    // --snip--
    .build();
let client = aws_sdk_s3::Client::from_conf(config);

or specify a custom chunk size

let config = aws_sdk_s3::Config::builder()
    .aws_chunked_encoding_chunk_size(Some(10240)) // 10 KiB chunks
    // --snip--
    .build();
let client = aws_sdk_s3::Client::from_conf(config);

Chunk signing

While this PR provides chunk signing capability, it only activates for non-TLS requests. For the majority of use cases where TLS provides encryption, streaming payloads continue to be sent unsigned with x-amz-content-sha256 set to STREAMING-UNSIGNED-PAYLOAD-TRAILER.

Implementation overview

The PR implements three key components:

1. Stream chunking: Breaks input streams into chunks
2. Deferred signer: Provides a signer specifically for aws-chunked encoding
3. Signer integration: Threads the signer through to AwsChunkedBody

Similar to event stream signing, chunk signing occurs after the initial HTTP request signing (sign_http_request) and requires context from that process. The deferred signer enables chunk signing to be wired up later as streaming bodies are yielded.

Testing

• Unit tests for int_log16; it had a bug for the input value 0 (should return 1 but had returned 0)
• Unit tests for stream chunking and chunk signing
• Integration tests for chunk signing and configurable chunk sizes
• CI

Checklist

• 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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[landonxjames]

LGTM! One question about an edge case with user-set chunk sizes and some other small notes, nothing blocking.

[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]

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

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[github-actions]

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

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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
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

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[aajtodd]

Overall looks good.

[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• 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.