Implement connection interface for mocking data

[furang]

What?

This request implement ConnectionInterface. I think this is very useful so now you can create custom connection service and mock necessary responses without even having developer account.
You just have to create a Client instance and set custom Connection service with setConnection

use Bigcommerce\Api\Client as Bigcommerce;

...

$customConnection = new ConnectionMock();

...

Bigcommerce::configure($creds);
Bigcommerce::setConnection($customConnection);

After that you can use any bundle methods and receive responses mocked in custom ConnectionMock class.