[JENKINS-37389] add @symbol("custom") to CustomTool's descriptor

The `@Symbol` annotation defines a unique identifier which is used by DSL. Whit this commit it is possible to distinguish a difference between tools when the the `tool` step is called. Example:
* tool name: 'myTool', type: 'jdk' <- returns JDK with id "myTool"
* tool name: 'myTool', type: 'custom' ← returns custom tool with id "myTool"

src/main/java/com/cloudbees/jenkins/plugins/customtools/CustomTool.java

@@ -53,6 +53,7 @@
 import jenkins.MasterToSlaveFileCallable;
 import jenkins.plugins.customtools.util.envvars.VariablesSubstitutionHelper;
 
+import org.jenkinsci.Symbol;
 import org.kohsuke.accmod.Restricted;
 import org.kohsuke.accmod.restrictions.NoExternalUse;
 import org.kohsuke.stapler.DataBoundConstructor;
@@ -211,6 +212,7 @@ public void check() throws CustomToolException {
     }
 
     @Extension
+    @Symbol("custom")
     public static class DescriptorImpl extends ToolDescriptor<CustomTool> {
 
         public DescriptorImpl() {