The backend for this lives in xdg-desktop-portal, and is in turn using GProxyResolver.
https://bugzilla.gnome.org/show_bug.cgi?id=768498
e362a01 Add a portalized proxy resolver implementation
gio/Makefile.am | 14 ++-
gio/giomodule.c | 2 +
gio/gproxyresolverportal.c | 167 ++++++++++++++++++++++++++
gio/gproxyresolverportal.h | 47 ++++++++
gio/org.freedesktop.portal.ProxyResolver.xml | 29 +++++
5 files changed, 255 insertions(+), 4 deletions(-)
Upstream: git.gnome.org