Allow customizing what fields are available in GravityView exports

[zackkatz]

This is handled nicely by GravityExport (especially once we implement #1891), but it's worth considering:

Customers often want to exports with all the entry data, not just configured fields.

We should add a setting to the View Settings, possibly with radio button:

• Only fields in the View configuration
• Enable all fields to be exported in the CSV

Or we could add field setting to turn on/off export in CSV?

Update: Perhaps we only implement this one: https://github.com/GravityKit/GravityExport/issues/119

[floatytheastronaut]

FreeScout #76560.
https://support.gravitykit.com/conversation/76560?folder_id=1

[rafaehlers]

This would be easier if this bug was fixed: https://github.com/GravityKit/DataTables/issues/179