Support for externalReferences.type source-archive

[prabhu]

Proposing to add a new URL type called source-archive to list the alternative permalinks for archives. This is particular useful when a VCS source gets removed without de-listing the packages. Example: caxa source code were removed from GitHub, while the packages continue to be available on npm.

Below is an example permalink for the cdxgen repo on Software Heritage.

https://archive.softwareheritage.org/swh:1:rev:068063aad71099eef383e0aeb35e88c001793906