Fix: Add support for custom headers in HTTP tool requests

[linznin]

Why are these changes needed?

The HttpTool in AutoGen accepts a headers parameter, but it is not being used in the actual request. This fix ensures that the headers provided by users are correctly included in HTTP requests. This resolves issues where authentication or other custom headers are required but currently ignored.

Related issue number

Closes #5638

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.