scripts/sbom: add JSON-LD serialization

scripts/sbom: add JSON-LD serialization

Add infrastructure to serialize an SPDX graph as a JSON-LD
document. NamespaceMaps in the SPDX document are converted
to custom prefixes in the @context field of the JSON-LD output.

The SBOM tool uses NamespaceMaps solely to shorten SPDX IDs,
avoiding repetition of full namespace URIs by using short prefixes.

Assisted-by: Cursor:claude-sonnet-4-5
Assisted-by: OpenCode:GLM-4-7
Co-developed-by: Maximilian Huber <maximilian.huber@tngtech.com>
Signed-off-by: Maximilian Huber <maximilian.huber@tngtech.com>
Signed-off-by: Luis Augenstein <luis.augenstein@tngtech.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>