Google Search sitelink assets support

Add support for creating and attaching sitelink assets to Google Search campaigns through POST /v1/ads/create.

Requested capabilities:

• Support at least four sitelinks.

• Accept text, final URL, description line 1, and description line 2.

• Attach assets to the campaign created in the same request.

• Return the created Google asset IDs.

• Report individual sitelink approval or rejection errors.

• Preserve Idempotency-Key behavior.

• Document character limits and required fields.

Until they implement it, sitelinks must be added manually in Google Ads. Our current integration is behaving correctly.