Creates a WebhookNotifier from the given webhook configuration.
Webhook URL and format (slack, discord, or plain).
Formats and posts an import summary to the webhook URL.
The ImportSummary to send.
Formats and posts an error notification to the webhook URL.
The error message string to include in the payload.
Formats and posts a plain message to the webhook URL.
The message text to include in the payload.
Webhook notification channel — posts import events as JSON to a configurable URL.