[FIX] partner_geolocalize_usability: avoid Nominatim HTTP 429 rejections

The stock ``base_geocoder._call_openstreetmap`` sends a generic Odoo
``User-Agent`` shared across thousands of instances, which Nominatim
rate-limits and blocks (HTTP 429 "Too many requests"), so mass
geolocation fails regardless of client-side throttling.

Override ``_call_openstreetmap`` to:

- send a deployment-specific ``User-Agent`` read from a new system
  parameter ``partner_geolocalize_usability.user_agent`` (shipped with an
  obvious placeholder so an unconfigured instance fails loudly instead of
  silently reusing the banned generic agent);
- throttle requests to one per second (``time.sleep``) before each call,
  complying with the Nominatim usage policy even when callers loop over
  many partners;
- return ``None`` instead of raising ``IndexError`` when Nominatim
  returns an empty result for an unknown address.

Document the required system parameter in the README and add tests
covering the User-Agent selection, throttling and empty/blank results.

partner_geolocalize_usability/__manifest__.py

@@ -17,10 +17,11 @@
     ],
     "qweb": [],
     "external_dependencies": {
-        "python": [],
+        "python": ["requests"],
     },
     # always loaded
     "data": [
+        "data/ir_config_parameter.xml",
         "views/res_partner.xml",
     ],
     # only loaded in demonstration mode