Expose encrypted data from the keyring

This makes it possible to request the encrypted data directly in application code. It will be used to download service account credential during "nomulus login".

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222847905

javatests/google/registry/testing/FakeKeyringModule.java

@@ -150,6 +150,12 @@ public final class FakeKeyringModule {
         return rdeReceiverKey;
       }
 
+      @Override
+      public String getEncryptedData(String keyName) {
+        throw new RuntimeException(
+            "Fake keyring does not support the retrieval of encrypted data.");
+      }
+
       @Override
       public void close() {}
     };