feat(requests): support custom amz headers

[overtrue]

Related issue

Closes #199

Background

Some S3-compatible services require custom x-amz-* request headers for bucket or object operations. The CLI did not expose a way to pass those headers.

Solution

• Add a global -H/--header NAME:VALUE option for x-amz-* request headers
• Inject custom headers before SigV4 signing for SDK-backed S3 requests
• Apply the same headers to manual XML S3 requests before signing
• Cover CLI parsing, help discoverability, and signed request behavior with tests

Validation

• make pre-commit

[Copilot]

Pull request overview

Adds a global -H/--header NAME:VALUE option that injects custom x-amz-* headers into both SDK-driven and manual XML S3 requests, signing them as part of SigV4.

Changes:

• New RequestHeader type plus set_global_request_headers/global_request_headers (process-wide OnceLock<RwLock<...>>) in rc-core, with parsing that enforces the x-amz- prefix and basic header validity.
• New global Clap arg request_headers parsed via RequestHeader::parse; execute populates the global before dispatch.
• S3Client::new reads the globals once, attaches a CustomHeaderInterceptor (modify_before_signing) for SDK calls and inserts headers into the manual XML signing path before sign_xml_request.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/core/src/alias.rs	Defines RequestHeader and global header storage with parse-time validation.
crates/core/src/lib.rs	Re-exports the new header types/functions.
crates/cli/src/commands/mod.rs	Adds the -H/--header global flag, value parser, and seeds the global registry; adds parse-tests.
crates/cli/tests/help_contract.rs	Asserts --header appears in help output.
crates/s3/src/client.rs	Adds CustomHeaderInterceptor for SDK requests, applies headers before SigV4 in the manual XML path, plus a new SDK-path test.
docs/reference/rc/README.md	Documents the new -H/--header global option.

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