Fix widget configuration schema

Dave Reidrequested to merge
issue/entity_browser-3250751:3250751-fix-widget-config-schema into 8.x-2.x
  • Added a new entity_browser.browser.widget.base config schema type for all widgets to extend from.
  • Fixed entity_browser.browser.widget.media_image_upload should extend entity_browser.browser.widget.upload.
  • Fixed UUIDs should use type uuid not string.

Closes #3250751

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