Feature/product attributes upgrade

[kidunot89]

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What does this implement/fix? Explain your changes.

• ProductAttribute changed from WPObjectType to WPInterface.
• LocalProductAttribute type implmented.
• GlobalProductAttribute type implemented.

Example Query

query {
  products(first: 20) {
    nodes {
      id
      name
      attributes {
        nodes {
          id
          attributeId
          name
          variation
          ... on LocalProductAttribute {
            scope
            options
          }
          ... on GlobalProductAttribute {
            scope
            options
            terms {
              nodes {
                name
                id
              }
            }
          }
        }
      }
    }
  }
}

Does this close any currently open issues?

#175

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