Load countries in wcSettings only when a block requires them

[Aljullu]

As discussed in #1574, since shippingCountries and availableCountries will be quite big once they include counties, we want to load them in wcSettings only when a block requires them. This PR implements that using the asset data registry.

How to test the changes in this Pull Request:

1. Create a post with an All Products block. Preview it and run console.log( wcSettings ); in the console. Verify it doesn't have shippingCountries neither allowedCountries.
2. Create a post with an Cart block. Preview it and run console.log( wcSettings ); in the console. Verify it does have shippingCountries but doesn't have allowedCountries.
3. Create a post with an Checkout block. Preview it and run console.log( wcSettings ); in the console. Verify it does have both shippingCountries and allowedCountries.

[mikejolley]

LGTM 👍 Think "3" you meant checkout but I tested it anyway :)

[Aljullu]

Think "3" you meant checkout but I tested it anyway :)

Right, fixed!