Add support for collection syncing.

[lukeholder]

Description

• Sync Collection just like we sync products.
• Allow the collection to return the related product elements.

[linear[bot]]

PT-1662 Add support for collection syncing.

[plcdnl]

We need it, please 🙏

[andrevvm]

This is a major deciding factor for us between using Craft or Sanity for Shopify projects. Any plans for the near future? Thank you!