New Sandbox API format

[rdumont]

This is required in order to comply with a breaking change in the Sandbox API.

The old format for POST /<vendor>/sandboxes/<sandbox>/<app>/files is a JSON array, such ass [ <change1>, <change2>, ... ].

Now it will be the following object.

{
  "account": "the account in context",
  "state": "an arbitrary state value, to be passed on to services through hooks",
  "changes": [ {change1}, {change2}, ... ]
}