Support non-project specific features to be installed on every devcontainer

[PureTryOut]

Is your feature request related to a problem?
Currently when using the official Devcontainer extension on VSCode I can configure features to be installed system-wide outside of any specific projects devcontainer.json. This allows me to install things into the container that I personally would like to have but don't want to bother other contributors with. In my case this is for example installing the Fish shell, since that's a very user-specific thing to have.
This seems to not be a thing in Devpod.

Which solution do you suggest?
Allow configuring features user-wide rather than per individual devcontainer.json. In VSCode I have the following in settings.json which accomplishes this, something similar would be nice to have for Devpod:

"dev.containers.defaultFeatures": {
    "ghcr.io/meaningful-ooo/devcontainer-features/fish:1": {}
}

The same thing with default extensions to install into editors btw, but to me the features is the most important one.

Which alternative solutions exist?
None that I know off, but perhaps I missed documentation somewhere.

[hiSandog]

The key contract is merge precedence between image/devcontainer features, project configuration, provider defaults, and the user's global features. Matching the Dev Containers behavior would be safest if feature IDs are normalized, project options override global options for the same feature, and users can explicitly disable a global feature for one workspace. Showing the final merged feature set in devpod up --debug or an inspect command would also make failures from personal configuration diagnosable for collaborators.

[skevetter]

I have added this feature. Check out my fork #1946

[PureTryOut]

Great to hear! Is there some documentation on how to use this? Where do I put what lines?