[Product Type Import]: Sync existing productTypes

### Support reference number

SUPPORT-3985

### Description

Currently the productType import tool is not syncing existing productTypes.

### Expected Behavior

Changes to existing productTypes can be done through the import tool.

### Current Behavior

Only new productTypes can be imported. We cannot add any new attribute to an existing productType.

Start at the productType import tool entry point and trace how existing productTypes are handled. Reproduce an import that adds an attribute to an existing productType, then add coverage in the import tool's tests and confirm that changes to existing productTypes are applied.

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