Require SSL certificate hash on login by default

Note that it's possible to set a config option to disable this functionality
on a per-environment basis (we're disabling it for sandbox), but in general
SSL certificate hashes should be required for increased security.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=225053496

java/google/registry/config/RegistryConfigSettings.java

@@ -92,6 +92,7 @@ public class RegistryConfigSettings {
     public String rdapTos;
     public String rdapTosStaticUrl;
     public String spec11EmailBodyTemplate;
+    public boolean requireSslCertificates;
   }
 
   /** Configuration for Cloud Datastore. */