feat: Create/Update changes for RSS feed notif channel support

[neilv-g]

The Read and Delete NotifChannel APIs should work as is and not need any changes for RSS feeds.

[jcscottiii]

LGTM with two nits to help. Feel free to do those now or in a follow up.

[jcscottiii]

See other note

[jcscottiii]

I think it's time we leverage the switch case (and get coverage from the exhaustive) linter. Feel free to do this in this PR or a follow up:

func validateUpdateNotificationChannel(request *backend.UpdateNotificationChannelRequest) fieldErrors {
	var fieldErrors &fieldValidationErrors{fieldErrorMap: nil} 

	if len(input.Name) < notificationChannelNameMinLength || len(input.Name) > notificationChannelNameMaxLength {
		fieldErrors.addFieldError("name", errNotificationChannelInvalidNameLength)
	}

	// 1. Get the explicit type discriminator from the JSON
	discriminator, err := request.Config.Discriminator()
	if err != nil {
		fieldErrors.addFieldError("config", errors.New("invalid configuration structure"))
		return fieldErrors
	}

	// 2. Switch on the type (cast to enum for exhaustiveness and type safety)
	switch backend.NotificationChannelType(discriminator) {

	case backend.NotificationChannelTypeRss:
		// Valid! RSS channels currently have no configuration fields to validate.
		// No variable is created, so no unused variable issue.

	case backend.NotificationChannelTypeWebhook:
		cfg, err := request.Config.AsWebhookConfig()
		if err != nil {
			fieldErrors.addFieldError("config", errors.New("invalid webhook config"))
			break
		}
		if err := httputils.ValidateSlackWebhookURL(cfg.Url); err != nil {
			fieldErrors.addFieldError("config.url", err)
		}

	case backend.NotificationChannelTypeEmail:
		// Explicitly fall through to default since email channels
		// cannot be managed manually via this endpoint.
		fallthrough

	default:
		fieldErrors.addFieldError("config", errors.New("invalid config: only webhook or rss updates are supported"))
	}

	if fieldErrors.hasErrors() {
		return fieldErrors
	}

	return nil
}