Allow specifying custom User-Agent globally for any & all http requests

[Azathothas]

Relevant homebrew discussion: https://github.com/orgs/Homebrew/discussions/5807

We need a global switch to toggle/override User-Agent header which would be used by any & all network requests
This, would be separate for soar dl, as we still need to have the ability to specify custom User-Agent only for soar dl operations but keep other http requests unchanged

[Azathothas]

Converted to an Issue: pkgforge/soar-dl#7