add download as stream functionallity

[kristian]

As a follow up to #50, it would be helpful to also be able to download artifacts as a stream, instead of always needing to download it into a file, before being able to process the file further (by reading it again and sending it elsewhere).

[stheine]

it looks like this would be rather easy to do, as there is already a stream https://github.com/jfrog/jfrog-client-js/blob/master/src/Artifactory/ArtifactoryDownloadClient.ts#L28-L31 and this could just be returned instead of passing this into the following fs.createWriteStream handling.

sorry, I can't create a PR on this, since I know too little of this code, nor do I speak typescript.