refactor: add extension config

[jmhbh]

Add extension config to main.go to allow extension configs to be provided directly to the controller. This allows downstream forks to easily extend the main controller with plugins and additional schemes using the following pattern

app.Start(func() (*app.ExtensionConfig, error) {
    return &app.ExtensionConfig{
        PluginFactories: []app.PluginFactory{
            func(client client.Client, scheme *runtime.Scheme, logger logr.Logger) transportadapter.TranslatorPlugin {
                myPlugin := NewMyPlugin(
                    logger.WithName("my-plugin"),
                    client,
                    scheme,
                )
                return myPlugin.Translate
            },
        },
        RegisterSchemes: func(scheme *runtime.Scheme) error {
            return myapigroup.AddToScheme(scheme)
        },
    }, nil
})