chore(gatsby-source-contentful): download assets via gatsby-core-utils

[axe312ger]

This replaces all custom asset download code with gatsby-core-utils from Gatsby v4.

Primarily as companion for #33461.

We might only merge this for now and see how it behaves, but I am pretty sure we at least need reload on 429s and 503s in core which AFAICS is not in Gatsby core yet.

[axe312ger]

The old code caches the base64 result files onto the file system. As we now use fetchRemoteFile, I am pretty sure we should be able to remove the following code and remove unnecessary I/O?

• gatsby/packages/gatsby-source-contentful/src/extend-node-type.js

  Lines 116 to 130 in c7a2cfc

  	// TODO: Find the best place for this step. This is definitely not it.
  	fs.mkdirSync(CACHE_IMG_FOLDER, { recursive: true })
  	const cacheFile = path.join(CACHE_IMG_FOLDER, urlSha + `.base64`)
  	if (fs.existsSync(cacheFile)) {
  	// TODO: against dogma, confirm whether readFileSync is indeed slower
  	const promise = fs.promises.readFile(cacheFile, `utf8`)
  	inFlightBase64Cache.set(requestUrl, promise)
  	return promise.then(body => {
  	inFlightBase64Cache.delete(requestUrl)
  	resolvedBase64Cache.set(requestUrl, body)
  	return body
  	})
  	}

• gatsby/packages/gatsby-source-contentful/src/extend-node-type.js

  Lines 153 to 162 in c7a2cfc

  	try {
  	// TODO: against dogma, confirm whether writeFileSync is indeed slower
  	await fs.promises.writeFile(cacheFile, body)
  	return body
  	} catch (e) {
  	console.error(
  	`Contentful:getBase64Image: failed to write ${body.length} bytes remotely fetched from \`${requestUrl}\` to: \`${cacheFile}\`\nError: ${e}`
  	)
  	throw e
  	}

Also, am I correct that there are two additional memory caches?

1. gatsby/packages/gatsby-source-contentful/src/extend-node-type.js

   Lines 99 to 103 in c7a2cfc

   	// Prefer to return data sync if we already have it
   	const alreadyFetched = resolvedBase64Cache.get(requestUrl)
   	if (alreadyFetched) {
   	return alreadyFetched
   	}

2. gatsby/packages/gatsby-source-contentful/src/extend-node-type.js

   Lines 105 to 109 in c7a2cfc

   	// If already in flight for this url return the same promise as the first call
   	const inFlight = inFlightBase64Cache.get(requestUrl)
   	if (inFlight) {
   	return inFlight
   	}

I am sure these have been added by @pvdz for a good reason. Can we run/check the related benchmarks again?