Make artifacts visible in Aim UI

[jgiannuzzi]

Once #6 and #224 are implemented, artifacts will become visible in the MLFlow UI.

This issue is about making them visible in the Aim UI. Aim treats artifacts like metrics and thus expects a few extra properties. Here is how I suggest we map them initially:

Aim	FML
name	file name
context	{}
step	0
epoch	0
timestamp	file timestamp

There is also limited support for artifact types. I think we only want to expose images and texts initially. We could detect the file's MIME type to distinguish between images and texts. The search possibilities will be quite limited and tricky unless we implement #225 and use it to store metadata in our database.

[Sharpz7]

This would be great to see folks. Thank you :))