Serialize fewer pipeline fields for MR widget

What does this MR do?

The motivation is a crazy number of SQL requests for widget.json call: !37246 (merged)

One of the problems is a missing inverse_of (I think so due to a large number of project calls)

We could optimize the number of requests or investigate which fields are used by FE and which aren't. Currently, widget.json returns detailed information about head pipelines and merge pipelines: !37246 (merged).json. We can reduce this information and return only the required fields based on: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/components/mr_widget_pipeline.vue

This merge request also introduces a feature flag which we can turn off if something is missed