Adds Customer to DownloadableItems connection

[kidunot89]

Your checklist for this pull request

Thanks for sending a pull request! Please make sure you click the link above to view the contribution guidelines, then fill out the blanks below.

🚨Please review the guidelines for contributing to this repository.

• Make sure you are making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Make sure you are requesting to pull request from a topic/feature/bugfix branch (right side). Don't pull request from your master!

What does this implement/fix? Explain your changes.

Adds a DownloadableItems connection the Customer type.

Usage example.

query {
    customer {
        downloadableItems(first: 10) {
            nodes {
                url
                accessExpires
                downloadId
                downloadsRemaining
                name
                product {
                    productId
                }
                download {
                    downloadId
                }
            }
        }
    }
}

Does this close any currently open issues?

Closes #233

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