Implement basic 'jmm models' command

[amisevsk]

Add basic implementation of jmm models command that prints artifacts in the local store. Prints digest, maintainer, format, and size for now.

Sample output:

❯ jmm models
DIGEST                                                                    MAINTAINER   MODEL FORMAT   SIZE
sha256:65784f6eae42940f917ed44b2f3f236f8a09b4a83064966eb081ef3aa2d47215   gorkem       onnx           28.9 MiB

Two concerns here, though:

1. I think we might me misusing oci.Store -- it's not entirely clear how index.json should work and if it supports multiple images at all. From my understanding, index.json is supposed to be the index for a single image (?).
2. Tests are still to come; since this code interacts with disk fairly heavily, I'll need to refactor the artifact.Store code to make it so we can sub out implementations in testing.

[gorkem]

I was using the function name as complete to do things like this.

[amisevsk]

I can rename it -- I chose this because complete was a little unclear to me ("complete from where?")

[amisevsk]

My assumption would've been that with viper/etc we don't need to do this manual wiring for every command (what's the point of a global option if its invisible until we explicitly assign it?) :)