feat: add FunctionalInterface annotation (#1515)

* feat: add `FunctionalInterface` annotation

* exclude internal api

* revise according to code review

* remove annotation on beta api

* remove annotation

* remove annotation

Author: JoeWang1127

api-common-java/src/main/java/com/google/api/core/ApiAsyncFunction.java

@@ -34,6 +34,7 @@
  *
  * <p>It is similar to Guava's {@code AsyncFunction}, redeclared so that Guava can be shaded.
  */
+@FunctionalInterface
 public interface ApiAsyncFunction<I, O> {
   /**
    * Returns an output Future to use in place of the given input. The output Future need not be

api-common-java/src/main/java/com/google/api/core/ApiFunction.java

@@ -34,6 +34,7 @@
  *
  * <p>It is similar to Guava's {@code Function}, redeclared so that Guava can be shaded.
  */
+@FunctionalInterface
 public interface ApiFunction<F, T> {
   T apply(F input);
 }

api-common-java/src/main/java/com/google/api/pathtemplate/ValidationException.java

@@ -39,6 +39,7 @@
  */
 public class ValidationException extends IllegalArgumentException {
 
+  @FunctionalInterface
   public interface Supplier<T> {
     T get();
   }

gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/FieldsExtractor.java

@@ -33,6 +33,7 @@
  * A functional interface to be implemented for each request message to extract specific fields from
  * it. For advanced usage only.
  */
+@FunctionalInterface
 public interface FieldsExtractor<RequestT, ParamsT> {
   ParamsT extract(RequestT request);
 }

gax-java/gax/src/main/java/com/google/api/gax/rpc/BidiStreamingCallable.java

@@ -100,15 +100,7 @@ public void call(final BidiStreamObserver<RequestT, ResponseT> bidiObserver) {
   /** Listens to server responses and send requests when the network is free. */
   public void call(
       final BidiStreamObserver<RequestT, ResponseT> bidiObserver, ApiCallContext context) {
-    internalCall(
-        bidiObserver,
-        new ClientStreamReadyObserver<RequestT>() {
-          @Override
-          public void onReady(ClientStream<RequestT> stream) {
-            bidiObserver.onReady(stream);
-          }
-        },
-        context);
+    internalCall(bidiObserver, bidiObserver, context);
   }
 
   /**
@@ -183,11 +175,8 @@ public ClientStream<RequestT> splitCall(
       ResponseObserver<ResponseT> responseObserver, ApiCallContext context) {
     return internalCall(
         responseObserver,
-        new ClientStreamReadyObserver<RequestT>() {
-          @Override
-          public void onReady(ClientStream<RequestT> stream) {
-            // no op
-          }
+        stream -> {
+          // no op
         },
         context);
   }

gax-java/gax/src/main/java/com/google/api/gax/rpc/ClientStreamReadyObserver.java

@@ -30,6 +30,7 @@
 package com.google.api.gax.rpc;
 
 /** A callback used to report that the {@link ClientStream} is ready to send more messages. */
+@FunctionalInterface
 public interface ClientStreamReadyObserver<V> {
   void onReady(ClientStream<V> stream);
 }