make --token optional when using --dry-run
In dry-run mode, we shouldn't have to require to have a gitlab token on public projects.
However, a couple of endpoints we are using require it:
- the list of labels associated to a project
- the history of labels associated to a MR/issue
For the first one, we can rely on the graphQL API to publicly fetch the data (not sure why one is public when the other is not).
For the second one, we can just paper over it because we are in dry-mode, so no changes will be done anyway.