Limit the automatic legend size to a set of static values

[markov00]

Is your feature request related to a problem? Please describe.
When showing multiple charts in a dashboard, it's hard to align the charts together due to various factors:

• the legend size depends on the content
• the chart area depends on the axes contents

Due to this content dependency, a nice alignment is hard to achieve. We already have add the ability to manually configure the legend width/height but we can improve it.

Describe the solution you'd like
The theme should support adding a set of values to which the legend should snap when computing its size automatically.
If we consider something like: [50, 100, 150, 200] in pixels, the resulting legend width should be the minimum value in this array that can contain the labels, or the biggest value if overflows it.

This is similar to this feature but for legend

Kibana Cross Issues
elastic/kibana#126018