Recommend evidence for `compositions.aggregate`

Currently, an SBOM could be declared as `aggregate=complete` without offering evidence. Ideally a suitable evidence must be presented under declarations.[evidence](https://cyclonedx.org/docs/1.6/json/#declarations_evidence) for such [claims](https://cyclonedx.org/docs/1.6/json/#declarations_claims), using the bom-ref as the link.

Currently, only an [author](https://cyclonedx.org/docs/1.6/json/#declarations_evidence_items_author) is allowed for evidence. This could be enhanced to support tools (components or services) since some of these declarations could be part automated.