Feature: Webhook integration in Acquia DAM for real-time asset sync

Package the entire webhook integration as a standalone submodule — acquia_dam_webhooks — so customers can opt in to real-time push events by enabling a single module. The submodule introduces a dedicated AcquiaDamWebhookClient service owning all 6 webhook API methods, while the parent acquia_dam module gains a new shared AssetMediaSyncService. When acquia_dam_webhooks is enabled, Acquia DAM pushes asset lifecycle events to Drupal, which processes the change immediately and synchronously — no cron delay.