官方文档：https://developer.fedex.com/api/zh-hk/catalog/upload-documents/v1/docs.html#operation/Image%20Upload%20Service%20Info
上传地址：https://documentapi.prod.fedex.com/documents/v1/lhsimages/upload
要注意下面的顺序，不按顺序会出错
第一个doucment
第二个attachment
IMAGE_1_request：
{"document": {"referenceId": "269414261346858582231", "name": "Alex.png", "contentType": "image/png", "meta": { "imageType": "SIGNATURE","imageIndex": "IMAGE_1"}},"rules": {"workflowName": "LetterheadSignature" }}
IMAGE_2_request：
{"document": {"referenceId": "269414261346858582232", "name": "LOGO.PNG", "contentType": "image/png", "meta": { "imageType": "LETTERHEAD","imageIndex": "IMAGE_2"}},"rules": {"workflowName": "LetterheadSignature" }}
