fix: DownloadableItem "product" field type changed to ProductUnion

[kidunot89]

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What does this implement/fix? Explain your changes.

• Changes DownloadableItem's product field type to ProductUnion to add support for product variation. Resolves "The \"variation\" product type is not supported by the core WPGraphQL WooCommerce (WooGraphQL) schema." #502

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Where has this been tested?

• WooGraphQL Version: …
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