Add meta-data artifact to the completion service

[karlmutch]

As an experimenter using the completion service
I want to be able to add an append only artifact for meta-data and control files
In order that experiment meta-data can be scrapped during runs and isolated, and the studioml eco-system can drop control and log files into it for tracing purposes

studioml/studio#357

studio itself should allow the addition of the _metadata artifact and from then on ignore it

#166
#165