feat: add CommonServiceConfig and general config handling based on viper

[aaronschweig]

This PR adds the ability to handle configuration in microservices centrally.

It basically extracts the main configuration points for a microservice in a CommonServiceConfig and generates cmd flags for it, which can be overriden from the env.

It also allows for extension via custom configuration for which also flags are generated.

It also sets up a viper instance ready to use for consumption with the configurations.

It can be used e.g.

// config.go
type Config struct {
	// CommonServiceConfig
	FGA struct {
		Target string `mapstructure:"fga-target"`
	} `mapstructure:",squash"`
	RuntimeKubeconfigPath      string `mapstructure:"runtime-kubeconfig-path"`
	APIExportEndpointSliceName string `mapstructure:"api-export-endpoint-slice-name"`
}

// service.go

var cfg config.Config
var v *viper.Viper

func init() {
	v = config.NewConfigFor(&cfg)

	cobra.OnInitialize(initConfig)

	initializerCmd.Flags().AddGoFlagSet(flag.CommandLine)

	v.BindPFlags(initializerCmd.Flags())
}

func initConfig() {
	// Parse environment variables into the Config struct
	if err := v.Unmarshal(&cfg); err != nil {
		log.Fatalf("Unable to decode into struct: %v", err)
	}

	if err := v.ReadInConfig(); err == nil {
		fmt.Println("Using config file:", v.ConfigFileUsed())
	}

	fmt.Printf("Loaded configuration: %+v\n", cfg)
}

[nexus49]

LGTM