Implement jwt_token_creds interop test

Using a JWT is a bit more work than it should be, but improving that
will come later.

At present, this test fails, but it is believed to be due to the auth
library.

interop-testing/src/main/java/io/grpc/testing/integration/AbstractTransportTest.java

@@ -48,6 +48,7 @@ import com.google.auth.oauth2.ComputeEngineCredentials;
 import com.google.auth.oauth2.GoogleCredentials;
 import com.google.auth.oauth2.OAuth2Credentials;
 import com.google.auth.oauth2.ServiceAccountCredentials;
+import com.google.auth.oauth2.ServiceAccountJwtAccessCredentials;
 import com.google.protobuf.ByteString;
 import com.google.protobuf.EmptyProtos.Empty;
 
@@ -817,6 +818,29 @@ public abstract class AbstractTransportTest {
     assertEquals(goldenResponse, response);
   }
 
+  /** Test JWT-based auth. */
+  public void jwtTokenCreds(InputStream serviceAccountJson) throws Exception {
+    final SimpleRequest request = SimpleRequest.newBuilder()
+        .setResponseType(PayloadType.COMPRESSABLE)
+        .setResponseSize(314159)
+        .setPayload(Payload.newBuilder()
+            .setBody(ByteString.copyFrom(new byte[271828])))
+        .setFillUsername(true)
+        .build();
+
+    ServiceAccountCredentials origCreds = (ServiceAccountCredentials)
+        GoogleCredentials.fromStream(serviceAccountJson);
+    ServiceAccountJwtAccessCredentials credentials = new ServiceAccountJwtAccessCredentials(
+        origCreds.getClientId(), origCreds.getClientEmail(), origCreds.getPrivateKey(),
+        origCreds.getPrivateKeyId());
+    TestServiceGrpc.TestServiceBlockingStub stub = blockingStub
+        .withInterceptors(new ClientAuthInterceptor(credentials,
+            Executors.newSingleThreadExecutor()));
+    SimpleResponse response = stub.unaryCall(request);
+    assertEquals(origCreds.getClientEmail(), response.getUsername());
+    assertEquals(314159, response.getPayload().getBody().size());
+  }
+
   /** Sends a unary rpc with raw oauth2 access token credentials. */
   public void oauth2AuthToken(String jsonKey, InputStream credentialsStream, String authScope)
       throws Exception {

interop-testing/src/main/java/io/grpc/testing/integration/TestServiceClient.java

@@ -167,6 +167,7 @@ public class TestServiceClient {
           + "\n      empty_stream: A stream that has zero-messages in both directions"
           + "\n      service_account_creds: large_unary with service_account auth"
           + "\n      compute_engine_creds: large_unary with compute engine auth"
+          + "\n      jwt_token_creds: JWT-based auth"
           + "\n      oauth2_auth_token: raw oauth2 access token auth"
           + "\n      per_rpc_creds: per rpc raw oauth2 access token auth"
           + "\n      cancel_after_begin: cancel stream after starting it"
@@ -230,6 +231,9 @@ public class TestServiceClient {
       String jsonKey = Files.toString(new File(serviceAccountKeyFile), Charset.forName("UTF-8"));
       FileInputStream credentialsStream = new FileInputStream(new File(serviceAccountKeyFile));
       tester.serviceAccountCreds(jsonKey, credentialsStream, oauthScope);
+    } else if ("jwt_token_creds".equals(testCase)) {
+      FileInputStream credentialsStream = new FileInputStream(new File(serviceAccountKeyFile));
+      tester.jwtTokenCreds(credentialsStream);
     } else if ("oauth2_auth_token".equals(testCase)) {
       String jsonKey = Files.toString(new File(serviceAccountKeyFile), Charset.forName("UTF-8"));
       FileInputStream credentialsStream = new FileInputStream(new File(serviceAccountKeyFile));