diracx-routers search endpoint relies on `preferred_username` instead of `sub`

Historically, the `JobDB.Owner` field corresponds to the owner name.
However, this approach is error-prone as we could potentially get multiple users with the same `preferred_username`.
The solution would be to depend on the `sub` which is the unique identifier in diracx.

Potential solutions that would need more thinking:
- we can add another attribute "Sub" in the DB, and diracx services would rely on that if given.
- keep it that way until the migration is done, then make a script to change from `preferred_username` to `sub`.