AM to cache, enumerate and publish arbitrarily documents listed in registry

alongside key-value configs, applications need to publish arbitrary text, XML &c documents.

The AM can do this, but needs to know what to publish and what metadata (mime type), serve up the artifacts, and support PUT operations pushing the artifacts up from agents, or have the agent provider enum & get the values.

We shouldn't need to persist this information, not if it is limited in size and rebuildable after restarts.