[MR Widget V2]: Handle action buttons

Now that !93017 (merged) is merged, we need to a placeholder for action buttons.

The implementation needs to look like the following:

Implementation plan

• https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/widget/widget.vue is where the action buttons will be implemented. We can follow the same implementation in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/extensions/base.vue#L338
• Note: The action buttons does not contain the Toggler on the mostright of the widget. That work will be captured here: #368613 (closed)