[discussion/proposal] supplemental provenance information

When building packages, especially on a team or in more complex environments, I often find it useful/necessary to have basic provenance information. It would be great if the following kinds of optional information were present in the schema supplement:

1. builder: who built the package
2. timestamp: when the package was build, presumably seconds since epoch
3. host/url: a machine identifier for where the package was built
4. build-tool: What tool actually built the package
5. build-tool-version: the version of the build-tool
6. tag/build-id: identifier for tracing the build
7. Others?

Since this is all optional, it would be up to the build tooling to decide whether and how they want to supply this information.

If this is of interest, I can take a stab at writing the additions to `schema-supplement.rst`