Merge pull request openmessaging#47 from llIlll/extend_builtinkeys

[ISSUE43]Add ACCOUNT_KEY to OMSBuiltinKeys

Author: duhenglucky

openmessaging-api/src/main/java/io/openmessaging/OMSBuiltinKeys.java

@@ -41,6 +41,11 @@ public interface OMSBuiltinKeys {
      */
     String ACCOUNT_ID = "ACCOUNT_ID";
 
+    /**
+     * The {@code ACCOUNT_KEY} key shows the specified account key in OMS attribute.
+     */
+    String ACCOUNT_KEY = "ACCOUNT_KEY";
+
     /**
      * The {@code REGION} key shows the specified region in OMS driver schema.
      */