[MR Widget V2] Add action buttons for widgets using extension api

Similar to #376152 (closed), we have to handle action buttons for widgets using the extension API. The difference between this issue and #376152 (closed) is that the other issue was for the MR Security Reports Widget, which uses the slot approach to render the component. This issue targets the widgets using the Extension API.

Implementation plan

• Extend https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/widget/widget_content_row.vue so that it displays action buttons on the right side of the row.

Verification steps

• Follow the steps to setup the feature locally: !103637 (merged)
• Verify that the MR widgets display action buttons on the secondary level as shown in the image above.