Add ability to extend Comment model via config

[adammparker]

Lots of popular/flexible models allow overridding the primary model (in this case Comment) with a downstream application model and setting that model in the packages config. This can allow for lots of things, for example, it would be nice if Spatie ActiivityLog could be added but is probably better for that to be downstream.

As part of this, it would be good if you can add additional "actions" like edit and delete to the right side of comments. Similar to how you can add actions to table rows in Filament with $table->actions. If the existing links were turned into actions and the HTML updated to render actions then you can let the user add their own actions like "Logs" to open the Activity Log side panel to see the history of edits on a comment.