fix: ProductUnion interface added

[kidunot89]

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What does this implement/fix? Explain your changes.

• ProductVariation no longer inheriting in the Product interface
• ProductUnion interface added. It's inherited by all Product types and ProductVariation type.
• Most common product connections are now resolving to the ProductUnion type.
• graphql_map_input_fields_to_wp_query hook added to Product_Connection_Resolver class. Resolves Problem with offset pagination and polylangs language filter parameter. #794

Does this close any currently open issues?

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Where has this been tested?

• WooGraphQL Version: …
• WPGraphQL Version: …
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