Support adding plugins from moodle.org/plugins

[abias]

As explained on https://moodlehq.github.io/moodle-plugin-ci/AddExtraPlugins.html, it is possible to add additional plugins to a moodle-plugin-ci build. Currently, only Git repositories and branches within these repositories are supported and this works fine.

However, if you want to add an additional plugin from a third party developer which is maintained in a rather "strange" way in git(hub) and the repo does not really offer a stable version in a particular git branch, it would be desirable to be able to add a published version of the plugin from moodle.org/plugin instead in the add-plugin configuration.

Alternatively, it might be sufficient for most of such third party plugins to be able to define a git tag instead of a branch in the add-plugin configuration.