MC-102: Add writer to send attributes to Magento

| Q                    | A
| -------------------- | ---
| Bug fix?             | n
| New feature?         | y
| Specs pass?*         | y
| Checkstyle issues?** | n
| PMD issues?***       | n
| Fixed tickets        | MC-102
| Doc PR               | n
Explain why pim_catalog_identifier is not in this mapping