Include custom headers in manifest when multipart upload is used

[tfius]

when uploading with HTTP can custom headers be added to manifest in a some form for example

 {
 	"entries": [{
 		"hash": "36b0615e60cc50efac8ba81dabced8ea805a68a6b803fda4b32206d13062aaaa",
 		"path": "data.json",
 		"contentType": "application/json",
 		"size": 138,
 		"mod_time": "2018-10-28T08:26:56.46376654Z",
 		"additionalData": {
 			"someDataHash": "hash",
 			"schemaUrlRef": "http://url",
 			"numRecords": 42,
 			"andSoOn": "AndOnAndOn"
 		}
 	}]
 }