fix(gatsby-source-drupal): process included nodes on preview and incremental builds

[pmelab]

Description

This PR addresses two issues:

1. When multiple nodes are updated at the same time via the preview or fastbuild updates, references to newly created content can get lost, depending on the order in the payload.
2. Fields that are marked as included within Drupal (with JSON:API Extras) are passed, but not processed by Gatsby in the preview or fastbuild updates.

Solution

I adapted handleWebhookUpdate to accept multiple nodes at once and process the references between them correctly to solve issue #1. Then I added included content from the json payload to the list of nodes to be processed on preview or fastbuild update.

[smurrayatwork]

@pmelab This PR has conflicts, FYI.