Http data issues and changes

Problem/Motivation

Drupal\migrate_plus\Plugin\migrate_plus\data_fetcher\Http::getResponse now requires returning a ResponseInterface.
This makes sense given that ::getResponseContent will call ->getBody() on the return from getResponse.

Solution

• ::getResponse calls the apitools client, either with changed methods or with a simple method.
• The apitools client methods need to be able to return array-like values as they are not intended to be methods specific to a migration http data fetcher.
• The return from an apitools client can now use an IterableResponse class.
• This will allow other implementations to iterate over the data results directly, while also allowing getResponse to return a ResponseInterface
• The getResponseContent will need to bypass use of getBody() and get the array that was embedded in IterableResponse.
• It will then need to re-encode this array to match the expectations of the data parser in the migration.