add ability to send HTTP header like user-agent or referer

[Strubbl]

Hi,

i would love to see graby being able to send additional http headers as configured in some ftr-site-config recipes.

Currently this is not supported:

graby/src/SiteConfig/SiteConfig.php

Line 39 in 026ffc7

// NOT YET USED

There are some site configs wanting to send the user-agent: https://github.com/fivefilters/ftr-site-config/search?utf8=%E2%9C%93&q=user-agent

An example is also wallabag/wallabag#2150 where the website thinks we are an internet explorer and we get redirected to hell.