Introduce simplified Default credential provision

As the first step in credential consolidation, we replace
injection of application default credential in for KMS and
Drive.

Tests:
- for Drive, tested with exportDomainLists and exportReservedTerms.
- For KMS, used CLI commands (get_keyring_secret and update_kms_keyring) to change and
  restore secret for one key.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=211819859

java/google/registry/config/CredentialModule.java

@@ -0,0 +1,68 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed 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 google.registry.config;
+
+import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
+import com.google.common.collect.ImmutableList;
+import dagger.Module;
+import dagger.Provides;
+import google.registry.config.RegistryConfig.Config;
+import java.io.IOException;
+import javax.inject.Qualifier;
+import javax.inject.Singleton;
+
+/**
+ * Dagger module that provides all {@link GoogleCredential GoogleCredentials} used in the
+ * application.
+ */
+@Module
+public abstract class CredentialModule {
+
+  /** Provides the default {@link GoogleCredential} from the Google Cloud runtime. */
+  @DefaultCredential
+  @Provides
+  @Singleton
+  public static GoogleCredential provideDefaultCredential(
+      @Config("credentialOauthScopes") ImmutableList<String> requiredScopes) {
+    GoogleCredential credential;
+    try {
+      credential = GoogleCredential.getApplicationDefault();
+    } catch (IOException e) {
+      throw new RuntimeException(e);
+    }
+    if (credential.createScopedRequired()) {
+      return credential.createScoped(requiredScopes);
+    }
+    return credential;
+  }
+
+  /** Dagger qualifier for the Application Default Credential. */
+  @Qualifier
+  public @interface DefaultCredential {}
+
+  /**
+   * Dagger qualifier for a credential from a service account's JSON key, to be used in non-request
+   * threads.
+   */
+  @Qualifier
+  public @interface JsonCredential {}
+
+  /**
+   * Dagger qualifier for a credential with delegated admin access for a dasher domain (for G
+   * Suite).
+   */
+  @Qualifier
+  public @interface DelegatedCredential {}
+}