Data Layer: Add helper to GET data from generic URL

See #28304

In this patch we're adding a small helper to be used to get raw
information from a given URL though a `GET` request.

The way we'd use this is inside of the `waitForData()` function. This is
meant to obscure the global http-data ID so that the user doesn't need
to worry about it.

```js
waitForData( {
	planets: getAtURL( 'https://swapi.co/api/planets/' ),
} ).then( ( { planets } ) => {
	console.log( planets.data );
} );
```

Author: dmsnell

client/state/data-getters/index.js

@@ -23,6 +23,11 @@ import {
 import { convertToSnakeCase } from 'state/data-layer/utils';
 import { dummyTaxRate } from 'lib/tax'; // #tax-on-checkout-placeholder
 
+export const getAtURL = url =>
+	requestHttpData( `get-at-url-${ url }`, rawHttp( { method: 'GET', url } ), {
+		fromApi: () => data => [ `get-at-url-${ url }`, data ],
+	} );
+
 export const requestActivityActionTypeCounts = (
 	siteId,
 	filter,