User mapping - Create relation contributors in Direct Transfer
To implement improved user mapping in Direct Transfer, the import side must know the names and usernames of original contributors/users to create placeholder users.
Currently, Direct Transfer doesn't export such information in the NDJSON files, and only the user IDs are exported.
So, in this issue, we will enable Direct Transfer to export the users' information.
Proposed solution
Create a new relation that exports an NDJSON file containing the user information, including the ID, name, and username of users that the user_id are exported in other NDJSON files.
Implementation ideas
Note: It would be best to extract data from all user IDs that could potentially be present in the exported NDJSON files. However, we could ignore contributions from a few tables, such as award_emoji,
as it wouldn't be interesting to create a placeholder user for a user who just added an emoji to a comment.