[feature] - Enable option to override request headers before proxying requests

[GabrielHorbach]

It is a straightforward change:

• Making the request available so the cookies can be accessed (and filtered!)
• Making a setRequestHeaders available so a header can be updated by adding it to the actions
• Updating the proxy flow so it checks for the request headers and adds them to the request

P.S.: Somehow, I couldn't get the correct indentation for the setRequestHeaders part, even though it looks correct in my branch.