support added for externally defined product type queries.

[kidunot89]

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What does this implement/fix? Explain your changes.

Adds single product queries for all product types enabled in WooGraphQL.

@jacobarriola Should probably take a look at this for product bundles.

Does this close any currently open issues?

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[jacobarriola]

@kidunot89 seems like this will dynamically define a RootQuery, eh? That's cool because I omitted that definition in my extension.