[RFC #0105] Exporter 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 exporter accepts -run instead of -stack and a new -extended flag. -extended is the location of the directory with extended run image layers. -run is the location of a run.toml file with schema:

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

When constructing the app image:

    - **If** image extensions were used to extend the `run-image` in [`analyzed.toml`](#analyzedtoml-toml):
      - MUST be based on the image in `<extended>`
    - **Else**:
      - MUST be based on the `run-image` in [`analyzed.toml`](#analyzedtoml-toml)

When finding run image mirrors, the exporter should use the appropriate value in run.toml.

The io.buildpacks.lifecycle.metadata label should be serialized according to the new schema defined in the spec PR.