Help Centre Import via API - Allow storing an external ID field against an article

(no functionality needed from the new field, to be used as reference only i.e. metadata)

I am importing a help centre using API, and I was beginning to get duplicates of my articles (because of multiple retries at transforming content). To solve this, I have been deleting articles with duplicate titles, but I realise now I may have a few articles with the same title in different collections.

It would be useful when creating or updating an article via API to be able to store a short string which references a unique identifier for the article on its old platform, so that I can be sure that each has been imported once and only once.