ObsCore Extension : how to implement it in a TAP service ?

[Bonnarel]

Currently our document is agnostic about the way to handle extension attributes in a TAP service.
Adding extension FIELDS in the Obscore table ? This is simple to do but may invalid a standard view on the main table
The alternative idea is to have the extension attributes as FIELDS in a second table. The open issue in that case was how to join that to the main table. The idea to use obs_publisher_did as a foreign key to do that didn't encounter consensus. However recent evolution of the JIVE service with different obs_publisher_did for each line of the table is a good indication that obs_publisher_did may be used to join the two tables in the future. I plan to write a PR with a new section on that.