New ability to create mirror and connect board columns via API

Hi ​@OmerK 

I’m really excited about this functionality - thank you :)

But I’m encountering issues when sending the defaults for board_relation columns as JSON strings. I first saw the issue when creating a column via Spring Boot’s GraphQLClient, and was able to replicate it in the API Playground, such that the column is created, but the defaults containing the target board IDs are not applied and the user has to select the connected board.

It does work in the API Playground if the defaults are sent as an object, so it appears to be a deserialisation issue on the API end.

Any thoughts?