[RFC #0105] Analyzer changes for run image extension (Dockerfiles phase 3)

[natalieparellano]

Spec PR: https://github.com/buildpacks/spec/pull/347/files

For newer platform API (0.12 and above), the analyzer accepts -run instead of -stack. -run is the location of a run.toml file with schema:

[[image]]
 image = "<image>"
 mirrors = ["<mirror1>", "<mirror2>"]

If no -run-image is provided, the analyzed run image should be the first image in run.toml.