[Feature] Generic Merge Request Report Widget

Proposal

Add a new generic merge request report widget that handles a new generic report type.

There are currently a multitude of merge request report widgets that handle specific cases and have their own logic applied. This however also means that if one wants to display a report from a custom job and none of the existing widgets handles the type of report that one wants to display there are only two options:

1. Use a report type that isn't really suited, such as code quality
2. Report it only as a pipeline artifact/fail a pipeline

This proposal would add a new merge request report type that takes generic reports from pipeline jobs (including child pipelines etc.) that follow the general format outlined in: https://docs.gitlab.com/ee/development/new_fe_guide/modules/widget_extensions.html#data-structure

This would especially mean no further processing would be needed and it could directly be displayed by the merge request widget.

This would also allow faster prototyping of new merge request report widgets and the addition of specific widgets by individual projects that collate relevant messages for that specific project.

I'm willing to work on a merge request regarding this feature if it is in general approved.

Related Issues:

#366214 (closed) #364823 (closed)