Adds configurable custom columns to table view

[alepar]

This is my shot at #755. It is mostly complete, just need to go through the rest of renderable types (PodWithMetrics,...) and make it implement RenderableRaw where possible. And make all of the parameters of HeaderColumn configurable via the settings file. Oh, and add it to the docs!
I'll finish that if the rest of the approach here is fine.

Some notable points:

• Configuration for this is currently in a separate file. I've considered putting it in views.yml instead (or another single file, where one could configure both view and model at the same time), but decided against it. Good argument can be made for both approaches, so I'd be happy to talk about it.
• I lack the knowledge to figure out how to test the GenericRenderer code path. The most questionable part is here. Would appreciate any pointers (essentially, which views use the code path). Will look around more.
• I don't think there's currently a way to sort a table on a custom column. But it is likely something that people will want.

Happy to discuss any other points.

[derailed]

@alepar Thank you for submitting this great PR! I haven't forgotten and generally like your approach here. I need to find the time to review and noodle on this feature. I think we should keep the column defs with the views imho to make this feature easier to manage.

[superbrothers]

I would like this feature. What is the status of this PR?

[Xyaren]

Is this still being worked on ?
I would really appreciate this feature.

[Champ-Goblem]

I would also be interested in this feature being added

[fragolinux]

very interested in this feature!

[cimlite]

I also think this is a great feature. Are there any news on it?
Is it possible to try it out in its current state, by cloning the repo and building it? Any additional configuration needed in that case?

Thanks!

[guanchzhou]

Looks like custom columns have been delivered in #3109.