Log when users alter params with value defined on plugin

We now log a message when users attempt to modify a PostgreSQL parameter whose value is already defined by a plugin (e.g. pgbackrest, powa, rsyslog). The log clearly indicates that the plugin defined value takes precedence and that the user change will not be applied.

To support this, plugin instance_settings() hook implementations now return a tuple composed of (plugin_name, Configuration) instead of just a Configuration object, allowing PostgreSQL configuration logic to identify the source of parameter definitions.

related to #505