feature: allow request object to set fileAttachment property

Calling the uploadFile operation requires the fileAttachment property to
be set correctly in the request object. Without the correct information
the uploaded file will not be found by the eBay API.

```php
$request->attachment('123ABC');
$request->fileAttachment = new Types\FileAttachment();
$request->fileAttachment->Data = '<xop:Include xmlns:xop="http://www.w3.org/2004/08/xop/include" href="cid:attachment.bin@devbay.net"/>';
$request->fileAttachment->Size = 6;

$response = $service->uploadFile($request);
```

In order to help users of the SDK the request object will assign the
correct values to the fileAttachment property when the uploadFile
operation is called.

```php
$request->attachment('123ABC');

$response = $service->uploadFile($request);
```
doc: update CHANGELOG