[FIX] base_geolocalize: allow to override user agent with system parameter

Before this commit, the useragent was harcoded as an Odoo one and if it was
blacklisted by the osmfoundation policy https://operations.osmfoundation.org/policies/nominatim/


all Odoo instances were forbidden to access the service.

This parameter allows to override the user agent with a system parameter.

closes odoo/odoo#134131

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>