Add an ability to import from github using multiple tokens

What does this MR do and why?

This MR adds a new optional param to GitHub Import API additional_access_tokens in order to supply the importer with more API keys (from different user/org accounts) in order to get rate limited less often (since a single token has 5k per hour limit) and as a result, speed the import process up.

Mentions #337232 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.