Simplified Asset Service to use Stream instead of FileStream

[VGrigoriev1991]

Simplified AssetService to use Stream instead of FileStream for file uploading since only the Stream base type is used further.

AssetService uses FileUploader for file uploading.
FileUploader expects Stream as first input parameter type when AssetService expects FileStream as first input parameter type.
AssetService doesn't use any features of the FileStream and can be simplified to use Stream as an first input parameter type.

This change should be useful in case of using Bynder SDK in clouds for serverless applications when there are limits on available disk space (for FileStream it's necessary to have file on the disk).

[coveralls]

coverage: 53.518%. remained the same
when pulling 84f7091 on VGrigoriev1991:asset-service-upload-file-stream
into 0324ec6 on Bynder:master.