Wrap all rate limiting logic inside GitHub API client
## What does this MR do?
Move the actual rate limiting logic to GitHub API to clean the code inside the GitHub importer, and avoid code duplication.
## Are there points in the code the reviewer needs to double check?
No there aren't.
## Why was this MR needed?
Avoid code duplication to handle API rate limit in every call to the GitHub API.
## What are the relevant issue numbers?
There are none.
## Screenshots (if relevant)
Not relevant.
See merge request !4552
