Add dagger map for injecting DnsWriter implementations

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

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

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;