Custom headers for all HTTP(s) requests

I'd like for Playwright to always pass a custom header for each HTTP(s) request it makes, either a fixed header, or a custom one. 

I want my backend server to know it's being accessed by Playwright so it can behave appropriately. In my case, I want to return LLM-optimized error messages when relevant (https://github.com/panozzaj/plain_errors)
