Allow basic support for gatsby-source-graphql

Problem/Motivation

On Gatsby Cloud it's not possible to use gatsby-source-graphql to trigger previews as it expects the Gatsby site to use gatsby-source-drupal. While gatsby-source-drupal is the recommended way, it should still be possible for gatsby-source-graphql to trigger a build. On top of this, other custom source plugins should be able to work for basic preview functionality (see https://www.drupal.org/project/gatsby/issues/3302739)

Steps to reproduce

Try used gatsby-source-graphql with this module on Gatsby cloud.

Proposed resolution

Add a custom header that Gatsby Cloud will use to allow defining which source plugin is used.

Remaining tasks

Need to add an option to the admin form for an optional source plugin name that is passed in as a header in all webhook requests.