Allow to retrieve artifact File Statistics

[tarioch]

Is your feature request related to a problem? Please describe.
It would be good to be able to retrieve File Statistics for an artifact. There is a rest endpoint for this, but it doesn't seem to be exposed through the java client.

Describe the solution you'd like to see
On ItemHandle provide a stats() that provides the stats according to the "File Statistics" rest api

Describe alternatives you've considered
At the moment the only way I found is to manually fire a rest call to the api.