Description
Access to JVM through passed-in GatewayClient during type conversion.
In customized type converters, we may utilize `jvm` field of passed-in GatewayClient directly to access JVM, rather than rely on the `SparkContext._jvm`.
That's [how](https://github.com/py4j/py4j/blob/master/py4j-python/src/py4j/java_collections.py#L508) Py4J explicit converters access JVM.