R1:57b0353034c5

Add `harbormaster.createartifact`

Summary:
Ref T8659. In the general case, this eventually allows build processes to do things like:

- Upload build results (like a ".app" or ".exe" or other binary).
- Pass complex results between build steps (e.g., build step A does something hard and build step B uses it to do something else).

Today, we're a long way away from having the infrastructure for that. However, it is useful to let third party build processes (like Jenkins) upload URIs that link back to…