Background block supports: add support for top-level styles in theme.json

[ramonopoly]

Allow setting site-wide background images in theme.json (top-level) styles.

Ticket to track the syncing of the following Gutenberg PRs:

​Global styles: background UI controls #59454

​Global styles: add background image to top-level theme.json styles #59354

​Background block supports: move block support defaults to gutenberg_render_background_support and revert gutenberg_get_background_support_styles #59889

​Global styles: force root min-height of 100% for backgrounds #59809

​Background images: remove required "file" prop #61387

There are multiple Gutenberg PRs because they iterate on the same feature.

WIP

Syncing Gutenberg PRs:

• ​Global styles: background UI controls #59454
• ​Global styles: add background image to top-level theme.json styles #59354
• ​Background block supports: move block support defaults to gutenberg_render_background_support and revert gutenberg_get_background_support_styles #59889