Add ReadbackOnce component to help one-shot readbacks. (#23845)

Users are sometimes confused about the fact that the `Readback`
component will fire multiple observers (once per every frame it exists
in the main world). This new `ReadbackOnce` component helps a bit with
the ux of spawning a readback that only fires once.

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Co-authored-by: Carter Anderson <mcanders1@gmail.com>