ThreadPoolBuilder add singlePool api.

hippo4j-core/src/main/java/cn/hippo4j/core/executor/support/ThreadPoolBuilder.java

@@ -147,6 +147,13 @@ public class ThreadPoolBuilder implements Builder<ThreadPoolExecutor> {
         return this;
     }
 
+    public ThreadPoolBuilder singlePool() {
+        int singleNum = 1;
+        this.corePoolSize = singleNum;
+        this.maxPoolSize = singleNum;
+        return this;
+    }
+
     public ThreadPoolBuilder poolThreadSize(int corePoolSize, int maxPoolSize) {
         this.corePoolSize = corePoolSize;
         this.maxPoolSize = maxPoolSize;