My try at creating the client extension (to be tested)

client/sql/0.0.1/00_header.sql

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+-- Complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION cdb_geocoder_client" to load this file. \quit

client/sql/0.0.1/05_geocoder_server_conf.sql

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+--
+-- This extension has its own table for configurations.
+--
+-- The table and the function are considered to be private and therefore
+-- no permissions are granted for any other user but the creator.
+
+CREATE TABLE IF NOT EXISTS _config ( KEY TEXT PRIMARY KEY, VALUE JSON NOT NULL );
+
+-- Needed to dump config in backups
+-- This can only be called from an SQL script executed by CREATE EXTENSION
+SELECT pg_catalog.pg_extension_config_dump('_config', '');
+
+
+CREATE OR REPLACE FUNCTION _config_set(key text, value JSON)
+RETURNS VOID AS $$
+BEGIN
+    PERFORM _config_remove(key);
+    EXECUTE 'INSERT INTO _config (KEY, VALUE) VALUES ($1, $2);' USING key, value;
+END
+$$ LANGUAGE PLPGSQL VOLATILE;
+
+
+CREATE OR REPLACE FUNCTION _config_remove(key text)
+RETURNS VOID AS $$
+BEGIN
+    EXECUTE 'DELETE FROM _config WHERE KEY = $1;' USING key;
+END
+$$ LANGUAGE PLPGSQL VOLATILE;
+
+CREATE OR REPLACE FUNCTION _config_get(key text)
+    RETURNS JSON AS $$
+DECLARE
+    value JSON;
+BEGIN
+    EXECUTE 'SELECT VALUE FROM _config WHERE KEY = $1;' INTO value USING key;
+    RETURN value;
+END
+$$ LANGUAGE PLPGSQL STABLE;

client/sql/0.0.1/10_admin0.sql

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+--
+-- Public geocoder API function
+--
+-- These are the only ones with permissions to publicuser role
+-- and should also be the only ones with SECURITY DEFINER
+
+CREATE OR REPLACE FUNCTION geocode_admin0_polygons(country_name text)
+RETURNS Geometry AS $$
+DECLARE
+  db_connection_str text;
+  ret Geometry;
+BEGIN
+  SELECT _config_get('db_connection_str') INTO db_connection_str;
+  SELECT _geocode_admin0_polygons(session_user, txid_current(), db_connection_str, country_name) INTO ret;
+  RETURN ret;
+END;
+$$ LANGUAGE 'plpgsql' SECURITY DEFINER;
+
+
+-- TODO: review all permissions stuff [I'd explicitly grant permissions to the public functions]
+
+--------------------------------------------------------------------------------
+
+CREATE OR REPLACE FUNCTION _geocode_admin0_polygons(user_id name, tx_id bigint, db_connection_str text, country_name text)
+RETURNS Geometry AS $$
+  -- TODO check if we can move the config to its own function
+  CONNECT db_connection_str;
+  SELECT geocode_admin0(user_id, tx_id, country_name);
+$$ LANGUAGE plproxy;

client/sql/00_installation_test.sql

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+-- Install dependencies
+CREATE EXTENSION postgis;
+CREATE EXTENSION plproxy;
+
+-- Install the extension
+CREATE EXTENSION cdb_geocoder_client;