Support for GitHub variables

Otterdog currently doesn't support variables.

Some of actions secrets don't need to be kept in pass or aren't sensitive, like gpg key id or other informational variables. Not having them clearly visible in otterdog config, causes additional issues being created by projects committers and further, debugging on our end.

Ref: https://eclipsefoundation.slack.com/archives/C04LA59B93M/p1701245889214829