Support for multiple bom files generation

[prabhu]

cdxgen could support generating multiple BOM files for a given project. We can support few styles of splitting and implement it as a postgen step.

Having such more granular representation for the ingredients of a digital product would help tiny ML models (~ 1M tokens) and tools like depscan v7, understand the composition layers better for deeper analysis. It would be super cool to have dependency-track support multiple bom file uploads per project too.

Splitting strategies

• By BOM type (OBOM, SaaSBOM, CBOM)
• By purl package type
• By lifecycle phase
• By analysis technique

We then need the following things:

• --out-dir argument to accept a directory, since the current output argument is a file.
• A naming strategy for the generated files, since the current default is bom.json.
• An index sbom file to link all generated bom files using externalReferences.
• Enhance the automatic composition logic to indicate that each bom file is incomplete.