Kibana version: 7.4.0

Describe the bug:

In the transforms wizard, the table to display index source data only displays a certain amount of fields by default and offers a dropdown menu to choose additional columns.

The transform preview table is missing this feature. For configuration with a lot of columns, this may results in quite unreadable and messed up table layouts, for example:

Expected behavior:

The table to preview a transform should have the same capabilities as the table for previewing source data.

Screenshots (if relevant):