Add possibility to load Video's poster image with fetchpriority="high"

[chrisvltn]

This is specially useful for Videos that are LCP, as recommended in web.dev:

You can improve the user experience in this case by adding a poster image. This gives the user some context on what the video is about. Additionally, if the poster image is your LCP element, you can increase the poster image's priority using the <link rel="preload"> hint along with fetchpriority="high":

<link rel="preload" as="image" href="poster.jpg" fetchpriority="high">

This could be done with a new posterFetchPriority prop in the Video component, so we could use it as:

<Video
  data={media}
  posterFetchPriority="high"
/>

And the component would render the link element along with the video element.