Autodiscover default credentials when nothing is explicitly specified.

This change reversed one made in PR #228 that broke automatic credential
discovery and instead replaced it with a "No Credentials" implementation.

Autodiscovery is a better default, and matches the behaviour documented
by the properties `gcs.credentials.path` and `gcs.credentials.json`
which state

> If not provided, the connector will try to detect the credentials
> automatically

Closes #291

src/main/java/io/aiven/kafka/connect/gcs/GcsSinkConfig.java

@@ -42,7 +42,6 @@
 import io.aiven.kafka.connect.common.templating.Template;
 
 import com.google.auth.oauth2.OAuth2Credentials;
-import com.google.cloud.NoCredentials;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.threeten.bp.Duration;
@@ -329,10 +328,6 @@ private void validate() {
     public OAuth2Credentials getCredentials() {
         final String credentialsPath = getString(GCS_CREDENTIALS_PATH_CONFIG);
         final Password credentialsJsonPwd = getPassword(GCS_CREDENTIALS_JSON_CONFIG);
-        if (credentialsPath == null && credentialsJsonPwd == null) {
-            LOG.warn("No GCS credentials provided, trying to connect without credentials.");
-            return NoCredentials.getInstance();
-        }
         try {
             String credentialsJson = null;
             if (credentialsJsonPwd != null) {

src/test/java/io/aiven/kafka/connect/gcs/config/GcsSinkConfigTest.java

@@ -23,6 +23,8 @@
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.mockStatic;
 
 import java.io.IOException;
 import java.net.URL;
@@ -50,13 +52,17 @@
 import io.aiven.kafka.connect.common.templating.Template;
 import io.aiven.kafka.connect.common.templating.VariableTemplatePart;
 import io.aiven.kafka.connect.gcs.GcsSinkConfig;
+import io.aiven.kafka.connect.gcs.GoogleCredentialsBuilder;
 
+import com.google.auth.oauth2.GoogleCredentials;
+import com.google.auth.oauth2.OAuth2Credentials;
 import com.google.auth.oauth2.UserCredentials;
 import com.google.common.io.Resources;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.NullSource;
 import org.junit.jupiter.params.provider.ValueSource;
+import org.mockito.MockedStatic;
 import org.threeten.bp.Duration;
 
 /**
@@ -377,6 +383,23 @@ void gcsCredentialsJson() throws IOException {
         assertEquals("test-client-secret", credentials.getClientSecret());
     }
 
+    @Test
+    void gcsCredentialsDefault() {
+        final Map<String, String> properties = Map.of("gcs.bucket.name", "test-bucket");
+
+        assertConfigDefValidationPasses(properties);
+
+        try (MockedStatic<GoogleCredentialsBuilder> mocked = mockStatic(GoogleCredentialsBuilder.class)) {
+            final GoogleCredentials googleCredentials = mock(GoogleCredentials.class);
+            mocked.when(() -> GoogleCredentialsBuilder.build(null, null)).thenReturn(googleCredentials);
+
+            final GcsSinkConfig config = new GcsSinkConfig(properties);
+
+            final OAuth2Credentials credentials = config.getCredentials();
+            assertEquals(googleCredentials, credentials);
+        }
+    }
+
     @Test
     void gcsCredentialsExclusivity() throws IOException {
         final Map<String, String> properties = new HashMap<>();