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.