[RFC #0095] Exporter should cache bom files for cached layers

[buildpack-bot]

This issue have been automatically created from pull request buildpacks/rfcs#166.

Given for example the following file structure:

/layers
  /config
    /sbom
      /cache
        /buildpack.id
          /cache-true-launch-true
            bom.cdx.json
          /cache-true-launch-false
            bom.cdx.json
        /other.buildpack.id
          /cache-true-launch-true
            bom.cdx.json
          /cache-true-launch-false
            bom.cdx.json

The exporter should create two layers - one for layers/config/sbom/cache/buildpack.id, the other for layers/config/sbom/cache/other.buildpack.id, and store them in the cache.

The io.buildpacks.lifecycle.cache.metadata label contains a list of buildpacks and their layers. The data for each buildpack should contain a SHA referencing the digest of the layer containing the boms for that buildpack.

[sambhav]

This should be closed now