Feature request: Allow setting padding to a specific display that are not main.

Currently, yabai has a feature to add a padding to main or all displays for widget spaces. Now I have a three displays, which is a MBP one, my main display, and another external one. Those displays are aligned like below.
```
_______________________
______|| |
| MBP || Main |
|______||______________________|
____________
| Sub |
|___________|
```

as you can see, Main is biggest and my main one, so I don't cutoff any space of that.
If I can add padding to Sub one, I can utilize 100% of my main display. Please consider adding support for multi displays like this.

Start by tracing yabai’s existing padding handling for main and all displays, then inspect how displays are identified and selected. Done means padding can be assigned to a specified non-main display while preserving current main/all behavior, including the three-display layout described.

Written by the indexing model from the issue text.