User story

As a Compass user
I want to select the fields that will be written into the exported JSON or CSV file
So that I only export what is needed and I can keep the file small

Acceptance Criteria

• Before exporting, the user can choose the fields that are exported (https://mongodb.invisionapp.com/share/BVNOZB4DYT3#/screens/349364209)
• When fields are selected/deselected, if a projection exists, it is modified according to the new selection
• Because the list of fields is probably generated from a sample of the collection, we give the user the possibility of adding other fields that may not exist in the sample
• When the export is completed, only the fields that the user selected are exported