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/dnsupdate/BUILD

@@ -21,6 +21,7 @@ java_library(
         "//third_party/java/jsr305_annotations",
         "//third_party/java/jsr330_inject",
         "//java/google/registry/config",
+        "//java/google/registry/dns/writer",
         "//java/google/registry/model",
         "//java/google/registry/util",
     ],

java/google/registry/dns/writer/dnsupdate/DnsUpdateWriter.java

@@ -25,7 +25,7 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.net.InternetDomainName;
 import google.registry.config.ConfigModule.Config;
-import google.registry.model.dns.DnsWriter;
+import google.registry.dns.writer.DnsWriter;
 import google.registry.model.domain.DomainResource;
 import google.registry.model.domain.secdns.DelegationSignerData;
 import google.registry.model.host.HostResource;
@@ -75,6 +75,12 @@ import org.xbill.DNS.Update;
  */
 public class DnsUpdateWriter implements DnsWriter {
 
+  /**
+   * The name of the pricing engine, as used in {@code Registry.dnsWriter}. Remember to change
+   * the value on affected Registry objects to prevent runtime failures.
+   */
+  public static final String NAME = "DnsUpdateWriter";
+
   private final Duration dnsTimeToLive;
   private final DnsMessageTransport transport;
   private final Clock clock;

java/google/registry/dns/writer/dnsupdate/DnsUpdateWriterModule.java

@@ -14,21 +14,26 @@
 
 package google.registry.dns.writer.dnsupdate;
 
-import dagger.Binds;
 import dagger.Module;
 import dagger.Provides;
-import google.registry.model.dns.DnsWriter;
+import dagger.multibindings.IntoMap;
+import dagger.multibindings.StringKey;
+import google.registry.dns.writer.DnsWriter;
 import javax.net.SocketFactory;
 
 /** Dagger module that provides a DnsUpdateWriter. */
 @Module
 public abstract class DnsUpdateWriterModule {
 
-  @Binds
-  abstract DnsWriter provideDnsWriter(DnsUpdateWriter dnsWriter);
-
   @Provides
   static SocketFactory provideSocketFactory() {
     return SocketFactory.getDefault();
   }
+
+  @Provides
+  @IntoMap
+  @StringKey(DnsUpdateWriter.NAME)
+  static DnsWriter provideDnsUpdateWriter(DnsUpdateWriter writer) {
+    return writer;
+  }
 }