Support for linking an existing bom with externalReferences

[prabhu]

Consider a project with the below structure.

Project
|---deps
|   |---- C dependency 1
|   |-----JS dependency 2
|   |-----Go dependency 3
|   |-----C dependency 4

cdxgen can currently generate a single comprehensive SBOM with some properties. However, what would be nice is to generate multiple SBOMs and one Parent SBOM.

Project
bom.json (Parent)
|---deps
|   |---- C dependency 1
|   |--------- bom.json
|   |-----JS dependency 2
|   |----------bom.json
|   |-----Go dependency 3
|   |----------bom.json
|   |-----C dependency 4
|   |----------bom.json

The parent SBOM could have external references linking to the individual SBOM using BOM-Link. One immediate benefit is that the parent SBOM would have a small number of components, so it would be easier to share.