Support video provider (like Youtube) as `background-video` for revealjs

Bug description

Hi!

I was trying to use background videos in revealjs but they do not appear.

Steps to reproduce

Below I am using a sample YouTube link provided in the Videos – Quarto docs (here with YouTube, also did not work with Vimeo link or local video):

````qmd
---
title: "Untitled"
format: revealjs
---

## {background-video="https://youtu.be/wo9vZccmqwc?autoplay=1"}

````

Expected behavior

Render slide with background video.

Actual behavior

Empty slide with no visible video.

Your environment

Tested:

• Remotely: Posit Cloud
• Locally:
  • macOS Sequoia 15.1
  • RStudio 2024.09.0+375

Quarto check output

Result of quarto check for Posit Cloud:

Quarto 1.5.57
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.2.0: OK
      Dart Sass version 1.70.0: OK
      Deno version 1.41.0: OK
      Typst version 0.11.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.5.57
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /usr/bin
      Version: 2019

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.8.17
      Path: /tmp/RtmplwAQRa/rstudio/terminal/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking R installation...........OK
      Version: 4.4.1
      Path: /opt/R/4.4.1/lib/R
      LibPaths:
        - /cloud/lib/x86_64-pc-linux-gnu-library/4.4
        - /opt/R/4.4.1/lib/R/library
      knitr: (None)
      rmarkdown: (None)

      The knitr package is not available in this R installation.
      Install with install.packages("knitr")
      The rmarkdown package is not available in this R installation.
      Install with install.packages("rmarkdown")