Trigger sync with Contentful webhooks

[danielaboost]

This PR introduces webhook-based syncing from Contentful.

Key changes include:

• Webhook endpoint & controller – added to handle incoming Contentful webhook events.
• SyncRunner service – encapsulates the logic for running syncs, triggered by webhook calls.
• Authentication & security – webhook requests require basic auth (with configurable credentials).
• Configuration options – supports setting sync mode (:poll or :webhook) via environment or initializer.
• Documentation – updated README with setup instructions (creating webhook in Contentful, configuring secrets, running sync).
• Testing & CI – added RSpec coverage for webhook handling

This allows Contentful to push changes to us in real time, rather than waiting for scheduled syncs. The gem also doesn't require rails - meaning the existing polling functionality works by default, and rails will need added to applications which want to use the webhook mode.

[richardmatthewsdev]

Exciting! Thanks @danielaboost :)