footer icons need HTML, which prevents extensions from resolving assets

Bug description

An extension defining a custom project type wants to declare a shared footer and use some images as icons within that footer. For example:

website:
  page-footer:
    right:
      - text: "<img src='/images/posit-logo-black-TM.svg' id='footer-right-posit-logo'>"
        href: "https://posit.co"

If the page footer supported an image attribute and took an image path, the in-extension path could be resolved without resorting to HTML.

website:
  page-footer:
    right:
      - image: "/images/posit-logo-black-TM.svg"
        href: "https://posit.co"

My understanding is that Quarto does not attempt to resolve asset paths within text, but can do so for fields which are known to contain file references.

Steps to reproduce

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Expected behavior

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Actual behavior

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Your environment

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Quarto check output

Quarto 1.4.551
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.11: OK
      Dart Sass version 1.69.5: OK
      Deno version 1.37.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.4.551
      Path: /Users/aron/quarto/quarto-1.4.551-macos/bin

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

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /Library/TeX/texbin
      Version: 2024

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

[✓] Checking Python 3 installation....OK
      Version: 3.12.2
      Path: /opt/homebrew/opt/python@3.12/bin/python3.12
      Jupyter: (None)

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

[✓] Checking R installation...........OK
      Version: 4.3.3
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.3-arm64/Resources/library
      knitr: 1.45
      rmarkdown: 2.26

[✓] Checking Knitr engine render......OK

Start by locating the page-footer configuration and the asset-resolution path used when rendering extension-defined website content. Compare the existing HTML text example with the proposed image field, then verify that an extension-relative image resolves correctly in the rendered footer.