utils/source-discoverer: Set the User-Agent

This avoids some web servers or WAFs doing weird stuff with our requests
which would lead to situations where our web calendar source discovery
would face server errors, redirects, incorrect contents, etc.

Fixes #651

Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/290

src/utils/gcal-utils.h

@@ -139,4 +139,6 @@ gboolean             gcal_is_valid_event_name                    (const gchar
 
 const gchar*         gcal_get_service_name_from_url              (const gchar        *url);
 
+SoupSession *        gcal_create_soup_session                    (void);
+
 #endif /* __GCAL_UTILS_H__ */