Software bill of materials (SBOM) generation as part of Heroku builds

As Heroku moves to Cloud-Native Buildpacks which [natively support generating a software bill of materials (SBOM)](https://buildpacks.io/docs/tools/pack/cli/pack_sbom_download/) artifact during the build process we'd like to capture this and surface it as metadata about the application, making it available via the API and CLI for fetching as needed.

We're also considering more advanced APIs that allow searching across SBOM data for multiple apps - but we'd like feedback on how it would be useful.