Check for product_variation post type for ACF relationship resolver

[jacobarriola]

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What does this implement/fix? Explain your changes.

This PR adds a check for the product_variation post type when attempting to resolve the ACF relationship field. ACF allows for product_variations, so we need to support this post type should it come through.

Does this close any currently open issues?

#470

Where has this been tested?

• WooGraphQL Version: 0.8.1
• WPGraphQL Version: 1.1.8
• WordPress Version: 5.7
• WooCommerce Version: 4.9.2

[kidunot89]

@jacobarriola The functionality this is trying to fix needs to be tested again to before the validity of the PR is confirmed.