URL path to the provider icon, as deployed at runtime, for example "/static/authgithub/github.svg" (in this
case "authgithub" is the plugin key. Source file is "src/main/resources/static/github.svg").
It can also be an external URL, for example "http://www.mydomain/myincon.png".
Must not be blank.
The recommended format is SVG with a size of 24x24 pixels.
Other supported format is PNG, with a size of 40x40 pixels.