libimage/manifests.list: hold artifact manifests

Add an AddArtifact() method which will craft an artifact manifest which
references one or more files and then add that manifest to the index.

When we need to build a reference to a list that includes our artifact
manifests, save the manifests and symlinks to their "layer" blobs along
with the contents of inlined blobs in a per-image directory, and add
references to those locations to the list of images we can search for
manifests and blobs.

Signed-off-by: Nalin Dahyabhai <[email protected]>