Thunderstore's main CDN regularly just fails to respond to requests.
Now that package list chunks are downloaded from the CDN, the issue
also affects this part of the program. While the only real fix is to
migrate to more reliable service provider, ETA on that is unknown.
Meanwhile do what can be done to mitigate the issue.

- Increase the number of retries and reduce the interval between them
  when loading the index chunk. Sometimes spamming the same request
  makes the CDN "wake up" and serve the file
- Add "preferred CDN" query parameter to index chunk request. If the
  Thunderstore API endpoint supports the received value, it will uses
  that CDN when returning the redirect response. Intercepting the
  redirect response to manipulate the redirect URL in the browser
  environment has proven hacky in the past and is avoided here
- Change the CDN on the fly on client when requesting the package list
  chunks. Since these request point directly to the CDN rather than
  Thunderstore API, manipulating them directly is feasible.