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

client/sql/0.0.1/05_geocoder_server_conf.sql

@@ -0,0 +1,38 @@
+--
+-- 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;