feat: Output the image id of an oci_load target.

[gonzojive]

Not the most elegant starlark, so this is a draft not meant for review, but it demos how to compute the image id.

Fixes #617

[thesayyn]

Thank you, nice to see you again :)

We'd want this to be subtarget that simply uses jq rule, also this is a breaking change since it adds more fields to defaultinfo which will break people who are using $(location) etc.