Allow customizing fields queried

[mglaman]

I'll help expand on this, but here's what I'm currently doing.

My config passes this to the theme plugin

      productFields:`id drupal_id title path { alias } body { processed }`,
      variationFields:`id drupal_id sku title price { number currency_code formatted } relationships { field_images { localFile { absolutePath childImageSharp { fluid(maxWidth: 555, maxHeight: 570) { base64 src aspectRatio src srcSet } } }}}`,

in my createPages

  let {productFields, variationFields} = pluginOptions;
  productFields = productFields || `id title path { alias } body { processed }`;
  variationFields = variationFields || `id sku title price { number currency_code formatted }`;

And then in the allBlahTypes

    if (nodeFields) {
      const nodes = await graphql(`{
              allCommerceProduct${productTypeNodeName} {
                nodes {
                  ${productFields}
                  relationships {
                    variations {
                      ${variationFields}
                    }
                  }
                }
              }
            }`);

I'll try to pop a PR based on this to expand the functionality with sourcing nodes here.