Allow to customize the Player UI more, e.g. the Play button #740

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Right now you can set controls for the Player and choose the default behavior, or you have the build the UI completely yourself.

We want to introduce a middle-way where you can override only certain elements of the Player control.

Acceptance criteria

• Allow at least two types of elements to be overridden, one of them being the Play/Pause button. Others can be: Fullscreen button, volume control, Time component, Slider
• The API could look something like this:

import { PlayButtonRenderMethod } from "@remotion/player"

// ...
const renderPlayPauseButton = useCallback(({paused}) => {
  if (paused) {
    return <CustomPauseButton />
  }

  return <CustomPlayButton />
}, []);

<Player renderPlayPauseButton={renderPlayPauseButton} />

• Add documentation to packages/docs/docs/player/api.md