Manage GitHub token for steps (and self-update)

[GideonBear]

Checklist

• I have searched the issue tracker for existing feature requests.

I want to suggest a feature

From #1474 (comment), #1474 (comment)
Topgrade should use a read-only (no perms) fine-grained GitHub access token supplied by the user for its own self-updating, and give it to steps that can use it (like deb_get and uv)

Contribution

• I am able and willing to implement this feature myself.
• I am willing to test if someone else implements it. Guide