Sentinel-2 download from Google Cloud Storage

[floeschau]

With this change, Sentinel-2 products can be downloaded from Google Cloud Storage.

The configuration or usersettings file needs to have a configuration section like this (the AccountFile and ProjectId keys contain the necessary authentication information):

{
    "Plugins": {
        "Terradue": {
            "Suppliers": {
                ...
                "GOOGLE": {
                    "Type": "Terradue.Stars.Data.Suppliers.DataHubSourceSupplier",
                    "ServiceUrl": "https://googleapis.com",
                    "AccountFile": "/path/to/google-account-file.json",
                    "ProjectId": "<project-id>",
                    "Priority": 3
                }
            }
        }
    },
    ...
}