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

[natalieparellano]

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

The restorer:

  - MUST resolve `run-image.reference` to a digest in `<analyzed>` if not present
  - MUST populate `run-image.target` data in `<analyzed>` if not present
  - **If** `<analyzed>` has `run-image.extend = true`, the lifecycle:
    - MUST download from the registry and save in OCI layout format the `run-image` in `<analyzed>` to `<kaniko-dir>/cache`

[natalieparellano]

We may only need the image manifest and config (like for build image extension). Will confirm as part of #998

[natalieparellano]

Edit: actually I don't think we need -generated, as analyzed.toml has all the info we need.