Plugin Proxy: Narrow data source config to DataSourceProxySettings
The data source proxy only needs a handful of fields from setting.Cfg.
Introduce a focused DataSourceProxySettings struct that exposes only
those fields, load Azure settings lazily via a resolver callback so the
proxy avoids touching Azure configuration unless an Azure-authenticated
route is matched, and pass the new struct through the datasource proxy
service in place of *setting.Cfg.
