Add ability to remove spacing in Dashboard panels

[streamich]

See: https://github.com/elastic/enhancements/issues/10626

And this: https://github.com/elastic/enhancements/issues/8071

Part of: #71857

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[nreese]

I though Dashboard already had this feature with "Use margins between panels" setting

[gose]

Here it is with margins turned off:

If we continue down the spectrum of "how dense can we make it", here's the extreme. It's akin to a page full of sparklines, where I'd need to mouse over any graph to see which host it represented and the axis values. This is a crude mockup, with a bad copy and paste job, but with real data the human eye would be able to quickly pick out any hosts straying from the pack. There are 720 hosts on the screen, all within eyeshot. Watching this many hosts is not ideal, I'd much prefer to turn Elastic ML on for these metrics and have it alert me on anomalous behavior. But many operators are in the situation of watching water boil, and the more information we can feed their eye, with one glance (no scrolling), the better.