Implement store.toml

[ekcasey]

Implement store.toml as outlined in the spec.

exporter

For each buildpack <bp.id>, that writes a <layer-dir>/<bp.id>/store.toml file during the build phase, exporter should add the JSON equivalent of the store.toml contents to a key named store in the corresponding buildpack map in the io.buildpacks.lifecycle.metadata label

analyzer

analyzer should read the previous image label and restore the contents of each buildpack's store to <layer-dir>/<bp.id>/store.toml.

analyzer should restore store.toml even when the -skip-layers flag is passed.

Examples

Example File:
/layers/bp.one/store.toml

[metadata]
  some-key = "some-val"

Example Label:
io.buildpack.lifecycle.metadata

{
  "buildpacks": [
    {
      "key": "bp.one",
      "version": "bp.one.version",
      "layers": {...},
      "store": {"metadata": {"some-key": "some-val"}}
   },
  ...
}