"DownloadableItem" type and connection added.

[kidunot89]

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What does this implement/fix? Explain your changes.

• Adds a new DownloadableItem type
• The downloadableItems field on the Order type resolves to a DownloadableItem connection.
• New test added DownloadableItemQueriesTest
• ConnectionPaginationTest updated.

Example Usage

query {
    customer {
        orders {
            nodes {
                downloadableItems {
                    nodes {
                        url
                        accessExpires
                        downloadId
                        downloadsRemaining
                        name
                        product {
                            productId
                        }
                        download {
                            downloadId
                        }
                    }
                }
            }
        }
    }
}

Does this close any currently open issues?

#218

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

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

…

Where has this been tested?

Operating System: Linux Mint 19.2

WordPress Version: 5.3.2