Add S3-style streaming validation

[dacut]

This adds a new function, sigv4_validate_streaming_headers, that is used to validate the headers of a streaming request. This is intended to be called after the headers have been read but before the body has been read, allowing for short-circuiting Expect: 100-Continue style requests when the headers are either malformed or incorrectly signed.

sigv4_validate_streaming_headers returns a new type, StreamingSignatureState, that has its own method, sigv4_validate_streaming_chunk, used to validate each chunk of an amz-chunked body. This must be called for each chunk, including the final, empty, terminating chunk.

I have not yet considered how this works with trailers.

[Copilot]

Pull request overview

Adds support for validating SigV4 “amz-chunked” streaming requests by splitting validation into an initial header check (before reading the body) and per-chunk signature verification.

Changes:

• Introduces sigv4_validate_streaming_headers and StreamingSignatureState::sigv4_validate_streaming_chunk for S3-style streaming signature validation.
• Refactors IntoRequestBytes into a new body module and exposes it at the crate root.
• Extends SignatureOptions with allowed_mismatch and adds/renames several SigV4/S3-related constants and error variants.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
src/signature.rs	Adds streaming header + chunk validation APIs; adds allowed_mismatch; adds new streaming tests.
src/canonical.rs	Adds from_request_and_body_hash to canonicalize without reading the body.
src/auth.rs	Adds validate_signature_with_key helper and exposes some internal fields for streaming state construction.
src/body.rs	New module containing IntoRequestBytes and basic implementations.
src/lib.rs	Wires in body module and re-exports it.
src/signing_key.rs	Adjusts doc link formatting and makes signing_key field pub(crate) for internal moves.
src/error.rs	Adds new SignatureError variants and mappings.
src/constants.rs	Adds new error codes/messages and S3 streaming-related constants; renames unsigned payload constant.
Cargo.toml	Adds streaming feature, http-body dependency, and optional tokio.

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