SBOM all the things

There was a [USA Cybersecurity Executive Order issued in 2021](https://www.whitehouse.gov/briefing-room/presidential-actions/2021/05/12/executive-order-on-improving-the-nations-cybersecurity/) that requires each software package supplied to the USA federal government to have a "Software Bill Of Materials" (SBOM):

> (j) the term “Software Bill of Materials” or “SBOM” means a formal record containing the details and supply chain relationships of various components used in building software. Software developers and vendors often create products by assembling existing open source and commercial software components. The SBOM enumerates these components in a product. It is analogous to a list of ingredients on food packaging. An SBOM is useful to those who develop or manufacture software, those who select or purchase software, and those who operate software. Developers often use available open source and third-party software components to create a product; an SBOM allows the builder to make sure those components are up to date and to respond quickly to new vulnerabilities. Buyers can use an SBOM to perform vulnerability or license analysis, both of which can be used to evaluate risk in a product. Those who operate software can use SBOMs to quickly and easily determine whether they are at potential risk of a newly discovered vulnerability. A widely used, machine-readable SBOM format allows for greater benefits through automation and tool integration. The SBOMs gain greater value when collectively stored in a repository that can be easily queried by other applications and systems. Understanding the supply chain of software, obtaining an SBOM, and using it to analyze known vulnerabilities are crucial in managing risk.

In order to enable the maximum distribution of libre fonts, I'd like to include a SBOM for each font family in the Google Fonts collection, with provenance and production details and signed with COSE Sign1; I wonder if we can finally make use of the DSIG table for real.

I'm assigning @simoncozens to start, and suggest to start with Noto CJK since that is one of the more difficult situations, since we don't build it ourselves, and its probably the most valuable font project we operate; and then integrate this into builder2 and the repo template.