[18052] GitHub importer - Scrape images to GitLab and update markdown (Issue/MR)

What does this MR do and why?

Implementation for a part of GitHub importer - Scrape images to GitLab and update markdown

This particular MR provides ability to import Issue description & MR description attachments.

feature flag

Screenshots or screen recordings

Local test

How to set up and validate locally

Enable Import Markdown attachments checkbox in Advanced import settings on GitHub import page. Create project via import from Github. The project should have Issues/MR-s with description that includes attachments. After import make sure that Issue/MR-s have attachment links that belongs to GitLab server.

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