Add possibility to send early hints headers

[jaysee]

It would be a great thing to be able to send HTTP 103 headers aka early hints using njs, actually, tryed using

r.headersOut['Link'] = '</style.css>; rel=preload; as=style';
r.headersOut['Link'] = '</script.js>; rel=preload; as=script';

r.return(103, "");

in js_content or when setting a variable with js_var...

but got error curl: (92) HTTP/2 stream 1 was not closed cleanly: PROTOCOL_ERROR (err 1)

makes sens, so needs internal support for this?

regards

[jo-carter]

@jaysee
nginx/nginx#326

You'd be able to do it using add_header in the location that has js_content at least if/when this merged.