[FEATURE]: Add external reference type for support policy/lifecycle metadata

## Describe the feature

While the work on CLE is progressing it would be useful to add an [`externalReference` type](https://cyclonedx.org/docs/1.6/json/#externalReferences_items_type) to hold a link to a CLE, OpenEOX or other kind of document that describes the versioning/support policy of the project.

## Possible solutions

Naming is certainly the most difficult problem, but the new type might be called `support-lifecycle` or simply `lifecycle`.

Currently there is no machine-readable format to describe EOS/EOL data, but the link could point to a human-readable page.