Epic: Shipping and billing address autocomplete in Block and Classic checkouts

[FangedParakeet]

Please see pca54o-8VN-p2 for more details on the requirements of this project.

The goal of this project is to provide autocomplete shipping and billing address fields in both blocks and shortcode checkouts. This functionality was explored and ultimately implemented in WooPay, commencing with Automattic/woopay#1700. We would like to integrate a similar solution enhancing WooPayments and the core checkout experience.

The high-level roadmap is that the core checkout team will develop support for autocomplete address fields in the core checkout experience, providing an interface for WooPayments to supply with responses from an API request to an external maps service (Google Place Autocomplete or the Google Autocomplete (new) API).

Checkout will continue to support browser and password manager autofill for contact info, addresses, and credit card information. For the shipping and billing address components, we’ll add support for predictive addresses using the Google Place Autocomplete or the Google Autocomplete (new) API combined with the Google Geocoding API.

The autocomplete API will be used with a geolocation bias to return predicted address results to the shopper as they type. When an address is selected, the Geocoding API is used to resolve the full address that will populate the address form.

To optimize cost and to bias to autofill which has higher usage, the address component including the first and last names are autofill enabled. In the address search field, the first two characters typed will return autofill results. On the 3rd character, autofill is disabled (no autofill UI) and a switch is made to autocomplete.

Again, the complete requirements for this integration lie within pca54o-8VN-p2.

This epic will track progress towards WooPayments' portion of this integration, specifically developing the API requests to Google's mapping service and providing these results to the interface implemented by the core checkout team. We will need to spike out whether these requests should be conducted from the client or server, how the request flow will tie into the provided core checkout interface, and how to manage API keys, among other tasks. As mentioned before, we should take as many of the learnings as possible from the integration in WooPay and apply them to our implementation here.