[WIP] Add prototype SBOM generator using Hatchling

[Copilot]

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Based on the design documents in design-docs directory, scaffold a prototype SBOM generator that use information from Hatchling (or work with Hatchling), with a roadmap to support setuptools in the future.

The prototype should be runnable.
Itself use hatch and hatchling for build.
Contains a small set of tests to demonstrate the prototype.

The prototype should try to build the source code from this
https://github.com/bact/sentimentdemo
and able to generate an SPDX 3 SBOM comparable to
https://github.com/bact/sentimentdemo/blob/main/bom.spdx3.json

(Note that the example SPDX 3 JSON may contain inaccuracies or lack of details. The generator which can access build information from the build backend is expecting to eventually produce a more accurate / more complete SBOM).

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