add shared search executor (#2312)

src/index/index.rs

@@ -322,6 +322,12 @@ impl Index {
         Ok(())
     }
 
+    /// Custom thread pool by a outer thread pool.
+    pub fn set_shared_multithread_executor(&mut self, shared_thread_pool: Arc<Executor>) -> crate::Result<()> {
+        self.executor = shared_thread_pool.clone();
+        Ok(())
+    }
+
     /// Replace the default single thread search executor pool
     /// by a thread pool with as many threads as there are CPUs on the system.
     pub fn set_default_multithread_executor(&mut self) -> crate::Result<()> {