[open-webui/open-webui] Fix issue #27427: support custom HTTP headers / Authorization for webhook configurations

Job description

Fix GitHub issue 27427 in open webui/open webui: https://github.com/open webui/open webui/issues/27427 Repo: https://github.com/open webui/open webui Target: Implement support for custom HTTP headers / Authorization in webhook configurations with a production ready upstream pull request. Required deliverables: A real upstream code change against open webui/open webui. An upstream pull request linked to issue 27427. Testing notes describing what you validated. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit the upstream patch plus the PR.