This makes it possible to add custom logic into how we map the document
data into the CSV data on a field-by-field basis.
- Allow custom data transformation to be added to
`custom.['plugin-import-export'].toCSV inside the field config
- Add type declaration to FieldCustom to improve types
- Export with `depth: 1`
Example:
```ts
{
name: 'customRelationship',
type: 'relationship',
relationTo: 'users',
custom: {
'plugin-import-export': {
toCSV: ({ value, columnName, row, siblingDoc, doc }) => {
row[`${columnName}_id`] = value.id
row[`${columnName}_email`] = value.email
},
},
},
},
```
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