Wire in domain transfer custom pricing and add a test

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142593949

java/google/registry/model/domain/fee/BaseFee.java

@@ -48,6 +48,14 @@ public abstract class BaseFee extends ImmutableObject {
     EAP("Early Access Period, fee expires: %s"),
     RENEW("renew"),
     RESTORE("restore"),
+    /**
+     * A transfer fee.
+     *
+     * <p>These are not used by the default system, in which the only fee associated with a transfer
+     * is the RENEW fee. These exist so that custom pricing logic can create a custom transfer fee
+     * if desired.
+     */
+    TRANSFER("transfer"),
     UPDATE("update"),
     CREDIT("%s credit");