feat: Add Sink.exists operator (#990)

* feat: +Flow.exists & Sink.exists

* chore: Update some doc and code.

* chore: Update method names in doc.

---------

Co-authored-by: He-Pin <hepin1989@gmail.com>

docs/src/test/java/jdocs/stream/operators/sink/Exists.java

@@ -0,0 +1,44 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package jdocs.stream.operators.sink;
+
+// #imports
+
+import org.apache.pekko.actor.ActorSystem;
+import org.apache.pekko.stream.javadsl.Sink;
+import org.apache.pekko.stream.javadsl.Source;
+
+import java.util.concurrent.TimeUnit;
+// #imports
+
+public class Exists {
+  private static final ActorSystem system = null;
+
+  private void existsExample() throws Exception {
+    // #exists
+    final boolean anyMatch =
+        Source.range(1, 4)
+            .runWith(Sink.exists(elem -> elem > 3), system)
+            .toCompletableFuture()
+            .get(3, TimeUnit.SECONDS);
+    System.out.println(anyMatch);
+    // Expected prints:
+    // true
+    // #exists
+  }
+}