feat: allow limiting column map in exports

[ralphjsmit]

Description

This PR adds the option to limit the ExportAction to certain columns. Previously, the only choice we had to limit columns was to have a column mapping form where the user can choose columns. However, sometimes we either want the column mapping form not to show all columns, or to not show a column mapping form and directly export a subset of columns.

This adds a ->columns() method that will allow the user to either:

• Specify which columns get shown in the column mapping form, if ->columnMapping(true) (default).
• Specify which subset of columns get outputted in the export without showing a form, if ->columnMapping(false).

Basically the four choices are now:

// Show column mapping form, include all columns

// Show column mapping form, include only specified columns
->columns(['id', 'full_name', 'email'])

// Hide column mapping form, export all columns
->columnMapping(false)

// Hide column mapping form, export specified columns
->columns(['id', 'full_name', 'email'])
->columnMapping(false)

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.

[zepfietje]

Been thinking about this before but didn't find the chance to work on it.
I think this is the missing piece for exports! 🚀