Add dagger map for injecting DnsWriter implementations

This is one of several CLs in a sequence for allowing per-TLD DNS
implementations.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129445641

java/google/registry/dns/writer/clouddns/CloudDnsModule.java

@@ -22,7 +22,10 @@ import com.google.api.services.dns.DnsScopes;
 import com.google.common.base.Function;
 import dagger.Module;
 import dagger.Provides;
+import dagger.multibindings.IntoMap;
+import dagger.multibindings.StringKey;
 import google.registry.config.ConfigModule.Config;
+import google.registry.dns.writer.DnsWriter;
 import java.util.Set;
 
 /** Dagger module for Google Cloud DNS service connection objects. */
@@ -39,4 +42,11 @@ public final class CloudDnsModule {
         .setApplicationName(projectId)
         .build();
   }
+
+  @Provides
+  @IntoMap
+  @StringKey(CloudDnsWriter.NAME)
+  static DnsWriter provideCloudDnsWriter(CloudDnsWriter writer) {
+    return writer;
+  }
 }