upload products to google merchant center via content api and handle disapprovals

Steps

1. Authenticate using a service account (for server-to-server) or OAuth 2.0 user flow with the `https://www.googleapis.com/auth/content` scope.
2. Insert or update a product by calling the `products.insert` method (or `products.update` for partial updates) with a product resource body; the `offerId` combined with `targetCountry` and `contentLanguage` forms the product ID.
3. For large catalogs, use `products.custombatch` to batch up to 1,000 product inserts/updates in a single HTTP request.
4. After insertion, call `products.get` or `products.list` and inspect the `destinations[].status` and `issues` array on each product to identify disapprovals and warnings.
5. Resolve disapprovals by correcting the specific attribute cited in the `issue.servability` and `issue.attributeName` fields, then resubmit the product.
6. Set up a data feed via the `datafeeds` resource as an alternative to the API for large catalogs that are already structured as delimited files.