F3 representation of issue/pull request labels

At the moment labels in issues and in pull requests are labels objects which include the id, color, etc, exactly as defined in the project. Not only is it a lot of duplicated information, it is also unnecessary because the APIs to modify the labels of an issue or a pull request only requires the label name in GitHub and in GitLab or the label id in Gitea.

The F3 schemas will be modified so the labels in issues and pull requests are a list of strings instead.