addToCart mutation

[kidunot89]

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What does this implement/fix? Explain your changes.

Adds addToCart mutation to schema.

mutation addToCart( $input: AddToCartInput! ) {
    addToCart( input: $input ) {
        clientMutationId
        cartItem {
            key
            product {
                id
            }
            variation {
                id
            }
            quantity
            subtotal
            subtotalTax
            total
            tax
        }
    }
}

AddToCartInput definition.

'clientMutationId' => array(
	'type'        => array( 'non_null' => 'ID' ),
	'description' => __( 'Unique ID for mutation', 'wp-graphql-woocommerce' ),
),
'productId'   => array(
	'type'        => array( 'non_null' => 'Int' ),
	'description' => __( 'Cart item product database ID or global ID', 'wp-graphql-woocommerce' ),
),
'quantity'    => array(
	'type'        => 'Int',
	'description' => __( 'Cart item quantity', 'wp-graphql-woocommerce' ),
),
'variationId' => array(
	'type'        => 'Int',
	'description' => __( 'Cart item product variation database ID or global ID', 'wp-graphql-woocommerce' ),
),
'variation'   => array(
	'type'        => array( 'list_of' => 'ProductAttributeInput' ),
	'description' => __( 'Cart item product variation attributes', 'wp-graphql-woocommerce' ),
),
'extraData'   => array(
	'type'        => 'String',
	'description' => __( 'JSON string representation of extra cart item data', 'wp-graphql-woocommerce' ),
),

Does this close any currently open issues?

…

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Where has this been tested?

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