[WIP] http (feature): Support adding attachment to the request

[Copilot]

• [ ] Analyze current HttpRequest/HttpMessage structure
• [ ] Add mutable attachment parameter to HttpMessage.Request
• [ ] Add attachment accessor methods to HttpMessage trait
• [ ] Ensure attachment is preserved when copying requests
• [ ] Create focused tests for attachment functionality
• [ ] Validate that existing functionality remains unchanged
• [ ] Test build and lint

The goal is to support adding context parameters (RPC context, logging context) to HTTP requests using a mutable attachment system while maintaining immutability for other request parameters.

Fixes #3880.

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