Export interface to add custom remote config provider

[wanliqun]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Can we export some method to register our custom remote config provider?

func getConfigManager(rp viper.RemoteProvider) (crypt.ConfigManager, error) {
	var cm crypt.ConfigManager
	var err error

	endpoints := strings.Split(rp.Endpoint(), ";")
	if rp.SecretKeyring() != "" {
		var kr *os.File
		kr, err = os.Open(rp.SecretKeyring())
		if err != nil {
			return nil, err
		}
		defer kr.Close()
		switch rp.Provider() {
		case "etcd":
			cm, err = crypt.NewEtcdConfigManager(endpoints, kr)
		case "etcd3":
			cm, err = crypt.NewEtcdV3ConfigManager(endpoints, kr)
		case "firestore":
			cm, err = crypt.NewFirestoreConfigManager(endpoints, kr)
		case "nats":
			cm, err = crypt.NewNatsConfigManager(endpoints, kr)
		default:
			cm, err = crypt.NewConsulConfigManager(endpoints, kr)
		}
	} else {
		switch rp.Provider() {
		case "etcd":
			cm, err = crypt.NewStandardEtcdConfigManager(endpoints)
		case "etcd3":
			cm, err = crypt.NewStandardEtcdV3ConfigManager(endpoints)
		case "firestore":
			cm, err = crypt.NewStandardFirestoreConfigManager(endpoints)
		case "nats":
			cm, err = crypt.NewStandardNatsConfigManager(endpoints)
		default:
			cm, err = crypt.NewStandardConsulConfigManager(endpoints)
		}
	}
	if err != nil {
		return nil, err
	}
	return cm, nil
}

func init() {
	viper.RemoteConfig = &remoteConfigProvider{}
}

Proposed Solution

Maybe we should export some method like RegisterRemoteProvider so that we can add our custom remote provider?

Alternatives Considered

No response

Additional Information

No response

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