Iceberg: Implement token authentication credentials

[tinchodias]

Implement token authentication credentials for using Iceberg with GitHub repositories. AFAIR, not only GH but also Gitlab and BitBucket support this kind of authentication using a token.

GitHub is deprecating password authentication. The problem and solutions for Pharo 8 and 9 are described here:
pharo-vcs/iceberg#1390

[guillep]

I think we need to update the Jenkins CI too, which is using password auth to get the repository's data to build.

[tinchodias]

Fixed by separate:
P8: #7732
P9: #7733