[388716] Github importer: collaborators (members)

What does this MR do and why?

Implementation for Import GitHub repository Collaborators as GitLab project members.

Screenshots or screen recordings

Local test 1 (simple mapping members from project to project)

Local test 2 (when some users has higher/lower role in target group)

In case of custom role (GitHub Enterprise Cloud)

How to set up and validate locally

1. Setup GitHub repository. Add few collaborators with different roles.
2. Make sure that those collaborators have public email.
3. Make sure that the same email users exists on GitLab side.
4. Run import from GitHub.
5. Check if roles mapping correct in project members.

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.