Add Free Trial support to Core Product Table / PDF Invoices

[jakejackson1]

Description
Gravity Forms payment add-ons like Stripe offer a Free Trial Subscription. When this is enabled, the Product Table generated by Gravity Forms will include a line item called "Free Trial" with a $0.00 value and in the subtotal area will include the text "Trial Discount" which zeros out the product total.

Gravity PDF doesn't currently support this feature when generating the Product Table or premium invoices.

Step To Reproduce
Steps to reproduce the behavior:

1. Install Stripe
2. Configure Stripe in Test Mode
3. Create a form with a product
4. Set up a Stripe Feed with a subscription and free trial enabled
5. Submit the form
6. View the Product table on the Entry Details page and/or Notification email
7. Set up a Core PDF on the form and view the PDF

Expected behavior
The Product table generated by Gravity PDF for Core and Universal templates should include the free trial information.

Support for this feature should be added to premium Invoice PDFs as well.