Query products by slug and sku

[kidunot89]

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What does this implement/fix? Explain your changes.

Adds slug and sku arguments to productBy query, with updated tests.

• ProductQueriesTest updated.

Example Usage

query ( $slug: String, $sku: String ) {
	productBy( slug: $slug, sku: $sku ) {
		...product fields
	}
}

Does this close any currently open issues?

…

Any relevant logs, error output, GraphiQL screenshots, etc?

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Any other comments?

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Where has this been tested?

Operating System: Ubuntu 18.04

WordPress Version: 5.2