spdx: allow comment in ExternalRef

[xnox]

Currently unused, but planning to generate an array of external refs, thus will want to document what they are for

References: https://spdx.github.io/spdx-spec/v2.3/package-information/#722-external-reference-comment-field https://github.com/spdx/tools-java/blob/master/testResources/SPDXJSONExample-v2.2.spdx.json#L80