Add configurable escrow file import GCS bucket

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=123760355

java/google/registry/config/RegistryConfig.java

@@ -91,6 +91,11 @@ public interface RegistryConfig {
    */
   public String getZoneFilesBucket();
 
+  /**
+   * Returns the Google Cloud Storage bucket for importing escrow files.
+   */
+  public String getEscrowFileImportBucket();
+
   /**
    * Returns {@code true} if TMCH certificate authority should be in testing mode.
    *