Charge invoice PDFs

The GET /api/1/dx/charging/history endpoint give us access to charging history.

In the response is: 
```
"invoices": [
    {
        "fileName": "XXXXXXXXX-GB.pdf",
        "contentId": "e32ec779-78ee-490c-bce6-XXXXXXX",
        "invoiceType": "IMMEDIATE"
    }
],
```

Can we access those pdfs? 