Add ContentType filter to gatsby-source-contenful

[xavivars]

When using gatsby-source-contentful, everything in the Contentful space configured in the plugin is downloaded, schemas are created, and nodes are cached.

While the first two steps (download + schema creation) are relatively fast (seconds), the node generation time can take a significant amount of time (mainly, if there's no previous cache, which may happen pretty often in some CI/CD pipelines).

It would be extremely good to have a plugin configuration option that allows us to filter out content types from even being processed.

A very naive (and probably incorrect, but good enough to make my point) implementation would be something like this (in https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-contentful/src/gatsby-node.js#L643)

// ADDED: NEW PLUGIN CONFIGURATION
const shouldProcessContentType = pluginConfig.get(`shouldProcessContentType`) || () => true

for (let i = 0; i < contentTypeItems.length; i++) {
   const contentTypeItem = contentTypeItems[i]

   // ADDED: FILTERING FOR PROCESSING NODES
   if (!shouldProcessContentType(contentTypeItem) {
       continue;
   }

   if (entryList[i].length) {
     reporter.info(
       `Creating ${entryList[i].length} Contentful ${
         pluginConfig.get(`useNameForId`)
           ? contentTypeItem.name
           : contentTypeItem.sys.id
       } nodes`
     )
   }

   // code continues....

A feature like this could decrease our build times significantly (in the minutes range): gatsby-source-contenful node creation is taking almost 10 minutes in some of our cases.

Would such a feature be welcome?