feat(gatsby, gatsby-source-contentful): add public action to disable stale node checks

[TylerBarnes]

Along with #37780 (+ more changes I'll pr soon) this allows a 4.9M node Contentful site, that used to need 64Gi memory, build with only 24Gi memory.

The previous code blocked the event loop and loaded all nodes into memory. Memory quickly grew linearly (similar to my comment in the linked PR). With these changes memory usage stays more flat, but also being able to opt out entirely makes stale node checks much faster. With contentful there are so many nodes where getNode needs to be called to get the parent node and check if it's stale and that's really slow/costly.

New api:

import { hasFeature } from "gatsby-plugin-utils"

exports.sourceNodes = ({ actions }) => {
  if (hasFeature(`stateful-source-nodes`)) {
    actions.enableStatefulSourceNodes()
  } else {
    // fallback to old behavior where all nodes are iterated on and touchNode is called.
  }
}