Feature Request: Add x-forward-header-* support for custom header forwarding

It would be valuable to have the ability to forward custom HTTP headers to target URLs through the Reader API. This would allow accessing content that requires specific authentication or custom headers.

The issue names no files, tests, or entry points. Start by tracing how the Reader API builds requests to target URLs and handles request headers. Done means headers using the x-forward-header-* convention are forwarded to target URLs, including cases requiring authentication or other custom headers.

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