SPDX 3.0 for ntia-conformance-checker

Google Summer of Code 2025 — adding SPDX 3.0 support to the tool that validates SBOMs against minimum-element requirements.

The ntia-conformance-checker validates SPDX SBOMs against the NTIA “minimum elements” and other baseline requirements.

As a Google Summer of Code 2025 contributor, I worked on adding SPDX 3.0 support to the checker, so that documents using the new SPDX 3 model can be validated alongside SPDX 2.