feat(query): adds contains key query operator

Author: koriwi

lib/mapping/q.js

@@ -61,6 +61,7 @@ class QueryAssignment {
  * @alias module:mapping~q
  * @type {Object}
  * @property {function} contains Represents the CQL operator "CONTAINS".
+ * @property {function} containsKey Represents the CQL operator "CONTAINS KEY".
  * @property {function} in_ Represents the CQL operator "IN".
  * @property {function} gt Represents the CQL operator greater than ">".
  * @property {function} gte Represents the CQL operator greater than or equals to ">=" .
@@ -80,6 +81,10 @@ const q = {
     return new QueryOperator('CONTAINS', value);
   },
 
+  containsKey: function containsKey(value) {
+    return new QueryOperator('CONTAINS KEY', value);
+  },
+
   in_: function in_(arr) {
     if (!Array.isArray(arr)) {
       throw new errors.ArgumentError('IN operator supports only Array values');