user_agent
Description:
User-Agent string to send with HTTP(S) requests, or null to use the default user agent, which is derived from
the geocode-glib version and GApplication:id
, for example: `geocode-glib/3.20 (MyAppId)`.
As per the Nominatim usage policy, it should be set to
a string which identifies the application which is using geocode-glib, and must be a valid [user agent](https://tools.ietf.org/html/rfc7231
section
-5.5.3) string.