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