Add actions to 'group' headers in tables

[dave-mills]

I think it'd be really useful to have more customisation options in the groups feature of tables. We often want to use the groups for cases where we want to display multi-level data in a single table - e.g: available survey forms, grouped by language. In this case, we want to have actions at the survey form level (e.g., to "deploy", "edit") and also at the language level("e.g. "edit", "add new survey").

I found another discussion asking about this: #13051, but haven't seen other questions about it, so maybe our use-cases aren't so common...

Possible Options

I looked at making a plugin for it, but it seems the Group class and the way the group headers are rendered are very much tied into the main Table class and blade templates, so I think it would be easier as a feature in core - is this something that could be included in V4? I'm happy to work on a PR for it if it doesn't conflict with other plans for table row groups.