Add downloadAttachments utility function

[liangyuanruo]

Part 2 following opengovsg/FormSG#1683.

The additional utility function on the FormSG SDK will be formsg.crypto.downloadAttachments(). This will take in the attachmentDownloadUrls, and return an object mapping fieldIds to an object containing the Blob and the original uploader filename.

downloadAttachments(map, secretKey) -> 
    Object(fieldId: String -> 
        Object(filename: String, data: Blob))