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

java/google/registry/keyring/api/InMemoryKeyring.java

@@ -149,6 +149,12 @@ public final class InMemoryKeyring implements Keyring {
     return jsonCredential;
   }
 
+  @Override
+  public String getEncryptedData(String keyName) {
+    throw new RuntimeException(
+        "In-memory keyring does not support the retrieval of encrypted data.");
+  }
+
   /** Does nothing. */
   @Override
   public void close() {}