Allow shop owners to just provide shop name instead of locate their own store ID.

[tw2113]

Majority of the users we're going to get are not technical/devs by nature. We should not require anyone to have to view-source on anything to set up the necessary credentials for our plugin here. Creating the API key is going to be cumbersome enough, but that one I know we can't get around. What we can get around is retrieving the store ID.

I propose we move things to providing a field to provide the shop name. "EndlessSkys" for an example from the forums. What we do is use that, and make a request to the following API endpoint, and retrieve the store IDs ourselves.

https://openapi.etsy.com/v2/private/shops/?shop_name=EndlessSkys&api_key=FILLMEIN

From the returned results, we should be able to pluck out the appropriate ID, and save that to meta like we already would be with the CMB2 field.

After that, we carry on like we had been. I see enough issues come through related to the API requests, that we need to make it more straightforward.