[ENH]: Configureable default padding around figure when using constrained_layout

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### Problem

Constrained layout figures fill out their frames too well, and adding padding during savefig makes the figures no longer adhere to the figsize given in my stylesheet, and isn't a by-default configured amount of padding. When these figures will be used (sometimes two half page-width figures side by side) in my latex document, the contents sit way too close. I don't want to change spacing within the subplot group -- that's all fine, I just want constrained layout to add space around the figure (incl. labels and maybe titles) while respecting the outer figure dimensions.

### Proposed solution

Add padding arguments to rcParams so that they may be configured on a stylesheet. These padding arguments should not influence the spacing of elements within the figure -- only around the figure.

Start by reviewing how constrained layout currently determines spacing around the figure and how rcParams are defined and applied. Establish how configurable outer padding can preserve the requested figsize without changing spacing within the subplot group, then add focused coverage for the proposed behavior.

Written by the indexing model from the issue text.