[dataimport] add a filter function to not fail if some extentity has several values for an attribute of final relation
the function will simply record a warning in the import log and keep a value
randomly.
Notice the function has to be explicitly inserted in the transformation stream.
[sources] synchronize source asynchronously when started from the UI
and redirect to the forthcoming import log, for a better user experience (there
is still a lot more to do there though).
Closes #10468967