Allow developers to explicitly declare "unsupported streams"

As came up in the office hours session this week, it would be helpful if developers could expose which API endpoints they explicitly have not covered yet in their implementation.

As Douwe mentioned in the video link, the Singer Spec does already support an "unsupported" flag for catalog metadata, and this could be applied if the developer wanted to expose to potential users (and also in SingerHub) which streams and covered by the tap and which streams are not covered.