[Wallet] Google Pay

[wjrosa]

Fixes #2707

Changes proposed in this Pull Request:

This PR introduces Google Pay as new wallet (regular, not an Express Checkout Element) payment method. This new method appears as a tab when selecting the "Credit / Debit Card" option during checkout.

The processing of transactions works just like the ECE version: the same popup appears and the shopper must select a card to pay.

This first PR do not add subscription support.

Testing instructions

• Checkout and build this branch on your test environment (add/google-pay-wallet-method)
• As a merchant, connect your Stripe account
• Confirm you can see Google Pay as a new regular payment method in settings (wp-admin/admin.php?page=wc-settings&tab=checkout&section=stripe&panel=methods):
  
• Enable the payment method
• As a shopper, access the test store from a public-acessible address using Chrome
• Add any product to your cart
• Go to checkout (both block and shortcode) and confirm you can see Google Pay when selecting the "Credit / Debit Card" option
  
• Confirm that you are able to complete the purchase (your email address must be included in the Google Pay testing group for the test cards to be available)

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

cc @diegocurbelo