Support config query

[hallyn]

There is a 'umoci config' command, which lets you set --config.entrypoint and --config.cmd. But near as I can tell you can only set, not query the current value. It would be nice to be able to query that value.

[hallyn]

(For now I will use 'umoci raw runtime-config --image oci:tag xx.json' and parse the intermediate file, but that's not ideal)

[cyphar]

Yeah. Ideally, umoci stat would have a better interface that would give you that sort of information. I haven't worked on umoci stat much (I mainly wrote it so that we could use it to test that the history was correctly modified) but it would be ideal if we could query things with high granularity using umoci stat.