Add support for pre-emptive authentication

[holyjak]

It would be great if the built-in auth also supported pre-emptive authorisation, i.e.sending the Authorization header at once, instead for waiting for the server to ask for it.

As everything, this is a pain in the HTTPClient. See https://hc.apache.org/httpcomponents-client-ga/tutorial/html/authentication.html 4.6. Preemptive authentication

Currently this can be done manually via http.setHeaders([Authorization: "Basic dG90dTI2Nzg6dG90dTI2Nzg="])