Allow arbitrary JSON to be added to artifact entries, and support search/filtering

[Enquier]

Right now you can commit an artifact blob to an element and have it added to the "_artifacts" of the element in question. It would be good to be able to add additional metadata about a particular blob/binary such-as:

"source": "magicdraw"

That way with filter on artifacts you could get an SVG that is produced by Tom Sawyer or another tool vs the one that was created by another tool.

Related is that the "/{extension}" artifact is non-deterministic in that it will return the first object with that extension that is found. For example if all your extensions were "tmp" it would return only the first one. A filter method would allow getting a specific binary based on the other metadata that was specified (like MimeType or "source" as per above)