Link the Registrar's RDAP server from RDAP domain replies

To do this we add a field of "rdapServers" in the Registrar object. Currently, we can only set this field manually, but a subsequent CL will add a cron-job to read these values from the ICANN servers.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=252438618

javatests/google/registry/testing/FullFieldsTestEntityHelper.java

@@ -80,6 +80,8 @@ public final class FullFieldsTestEntityHelper {
       .setFaxNumber("+1.2125551213")
       .setEmailAddress("contact-us@example.com")
       .setWhoisServer("whois.example.com")
+      .setRdapBaseUrls(ImmutableSet.of(
+          "https://rdap.example.com/withSlash/", "https://rdap.example.com/withoutSlash"))
       .setUrl("http://my.fake.url")
       .build();
   }