[JENKINS-68137] Enable custom icon symbols for plugins (#6395)

Co-authored-by: Ullrich Hafner <ullrich.hafner@gmail.com>
Co-authored-by: Tim Jacomb <timjacomb1+github@gmail.com>

core/src/main/java/hudson/Functions.java

@@ -2320,6 +2320,26 @@ public class Functions {
         return iconMetadata;
     }
 
+    @Restricted(NoExternalUse.class)
+    public static String extractPluginNameFromIconSrc(String iconSrc) {
+        if (iconSrc == null) {
+            return "";
+        }
+
+        if (!iconSrc.contains("plugin-")) {
+            return "";
+        }
+
+        String[] arr = iconSrc.split(" ");
+        for (String element : arr) {
+            if (element.startsWith("plugin-")) {
+                return element.replace("plugin-", "");
+            }
+        }
+
+        return "";
+    }
+
     @Restricted(NoExternalUse.class)
     public static String tryGetIconPath(String iconGuess, JellyContext context) {
         if (iconGuess == null) {