Add a label to the S3User's Secret to allow label matching and speed up fetching the secret

[phlg]

As of current version, the user controller lists every secret in a namespace to find the one secret matching a S3User, which is fairly inefficient :

s3-operator/controllers/user_controller.go

Lines 408 to 438 in 57a7f12

	err := r.List(ctx, secretsList, client.InNamespace(userResource.Namespace))
	if err != nil {
	logger.Error(err, "An error occurred while listing the secrets in user's namespace")
	return userSecret, fmt.Errorf("SecretListingFailed")
	}
	if len(secretsList.Items) == 0 {
	logger.Info("The user's namespace doesn't appear to contain any secret")
	return userSecret, nil
	}
	// In all the secrets inside the S3User's namespace, one should have an owner reference
	// pointing to the S3User. For that specific secret, we check if its name matches the one from
	// the S3User, whether explicit (userResource.Spec.SecretName) or implicit (userResource.Name)
	// In case of mismatch, that secret is deleted (and will be recreated) ; if there is a match,
	// it will be used for state comparison.
	uid := userResource.GetUID()
	// cmp.Or takes the first non "zero" value, see https://pkg.go.dev/cmp#Or
	effectiveS3UserSecretName := cmp.Or(userResource.Spec.SecretName, userResource.Name)
	for _, secret := range secretsList.Items {
	for _, ref := range secret.OwnerReferences {
	if ref.UID == uid {
	if secret.Name != effectiveS3UserSecretName {
	return secret, fmt.Errorf("S3UserSecretNameMismatch")
	} else {
	userSecret = secret
	break
	}
	}
	}
	}

This could benefit from a dedicated label add to the secret when it's created. This is not difficult in itself, but requires some thought regarding pre-existing secrets (as in : Should the operator reconcile secrets to add labels ? Should this be managed with a small one-shot script to add the label to every S3User secret ?)

EDIT : usage example visible in Operator SDK doc