Extend Copy to all kinds of archives and assets

[thomas11]

#423 gave the Copy resource the ability to take assets and archives as input, but only the path-based kinds. With custom handling, we could support the other kinds, too:

• StringAsset: write the string into the remote file.
• RemoteAsset and RemoteArchive: download first, then handle like a path asset.
  • It would be more efficient to SSH into the target system and initiate the copy from there (avoiding the relay), but that could introduce more failure types and be more complex to implement and test.
• AssetArchive: recurse through and do one of the above for each one.