Introduced ability to transfer issues from private to public repos

[Fgerthoffert]

Hello,

Thanks for the original work on the action.

This PR provides three new features I needed for my own workflows:

• Ability to create labels in the destination repository if missing (based on an available parameter in GitHub GraphQL API)
• Ability to transfer issues based on label names, and not only the destination defined in the router (for example attaching the label transfer:my-new-repo transfers the issue to my-new-repoif it exists.
• Ability to transfer issues from private repositories to public repositories.

That later one is a bit cumbersome as it involves creating a temporary repository, but it seems aligned with a suggested workaround found in various places.

I included documentation on how to use these features.

In the suggested PR, all these features are disabled by default. I tested them in my repositories (manual tests, no automated tests)

I understand this PR contains quite a bit of change and the implementation for moving issues from private to public is debatable.