Add support for plugin groups

Now supporting pluginGroups

Configuration

- name: maven-settings-xml-action
  uses: whelk-io/maven-settings-xml-action@v14
  with:
    plugin_groups: '[ "some.plugin.group.id", "some.other.plugin.group.id" ]'

Output

<settings>
  ...
  <pluginGroups>
    <pluginGroup>org.eclipse.jetty</pluginGroup>
  </pluginGroups>
  ...
</settings>