Add UTM parameters to product URLs in feed.

[budzanowski]

Adding UTM parameters to the products URLs in the feed

With this change the order attribution feature in
WooCommerce will be able to correctly attribute
purchase coming from the Pinterest platform.

Changes proposed in this Pull Request:

Each link in the product feed will have the UTM parameteres added:

-'http://example.org/?product=dummy-product'
+'http://example.org/?product=dummy-product&utm_source=pinterest&utm_medium=social'

Closes https://github.com/woocommerce/woocommerce-analytics/issues/497

Detailed test instructions:

1. Wait for the feed creation or trigger it manually
2. Inspect the created XML feed file.

Additional details:

• this may require followup in WooCommerce Analytics backend

Changelog entry

Add - UTM parameters to the products URLs used in the product feed.

[layoutd]

LGTM 🚀