Support getting/setting of Help Center article's content_tag_ids field

[andy-may-at]

Help Center articles have a (mutable & optional) 'content_tag_ids' field
The documentation says that the field contains 'The list of content tags attached to the article'

This field is not currently exposed in the Article model & so cannot be read/written
Feature request is to add it to the model.

Additional context
N.B. a potential risk of adding a new field to the model seems to be that any existing code using the client that was doing a overwrite of an existing article (by calling Zendesk.updateArticle(article) (without having recently read the article in question from the API, or where the Article instance has been created by copying a known set of fields from another Article instance) would have been preserving any pre-existing content_tags_id value on the article.
But if we add the field to the model, then any code like this will now be updating the content_tags_id field to be empty
This edge-case change in behaviour seems worthwhile, but may be worth documenting in release notes.

I'm building a PR for this issue & will submit it shortly