Allow any URI that gives SVG/PNG image to be used for plugin icon

[vitaliy-guliy]

Describe the bug

I'm adding Che Buildkit plugin #16539, which adds the buildkit-base container to Che workspace.
But there is no special logo image for the buildkit. In this case it's a good option is to use the organization logo.

Organization: https://github.com/moby
Buildkit repository: https://github.com/moby/buildkit

For that, we need to disable checking of the plugin image and use the value form plugin meta.yaml as it is.

If the plugin meta.yaml refers to a non-existent image, the frontend application should use the default one. I think that handling of such kind of errors is already implemented in Dashboard and Che-Theia.

Connected with #14502