User-Agent HTTP header is no longer sent in the requests
- as Jetty/9.4.20.v20190813 is blacklisted server-side.
The backported fix uses a shim wrapper for HttpUtil, allowing to inject
a customized Jetty's HttpClient, while keeping the rest of the interface
intact (goal was to minimize changes: 3.x branch has a more elegant fix
for the same, but coupled with other changes and refactorings that are
not part of this backport).
Resolves#8554
Signed-off-by: Mateusz Bronk <bronkm+gh@gmail.com>