feat(wcsg): respect checkbox label option in WCSG settings

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

There's an option in the WooCommerce Subscriptions Gifting (WCSG) extension that lets admins set the label of the gifting checkbox. This PR allows non-donation modal checkouts to show this label instead of a hardcoded label. The label is still hardcoded for donation checkouts, as the WCSG extension doesn't provide a filter for adding new custom options.

How to test the changes in this Pull Request:

Follow testing instructions in Automattic/newspack-plugin#3747

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[naxoc]

I reviewed this as part of #3747 and the modal worked great with this 👍