Allow specifying the upload provider type in the batch details

Situation

Currently, the Python SDK has 2 types of uploaders: one for Nuxeo and one for S3.
Which one to use is determinated through the provider retrieved from batch details.

Problem

Customers might implement a custom upload provider, with a name different from "s3" for instance, but still wanting to use the S3 uploader (think of alias).
This will make the Python SDK to always choose the default upload provider because it does a simple comparison against predefined provider names.

Solution

Customers can use aliases to force the selection of a given provider by adding the provider_type key to the extraInfo sub-dict of batch details.
Example:

{
    "provider":"custom",
    "fileEntries":[],
    "batchId":"...",
    "extraInfo":{"bucket":"...", "...", "provider_type": "s3"}
}