Move ping client into cache client in node sdk and add lazy loading in node and web sdks

[pgautier404]

In #516, we pull the ping client into the cache client and expose ping as a cache client method for the Web SDK. The same needs to be done for the nodejs SDK, and the functionality needs to be extended for both SDKs such that the ping client is lazy-loaded when it is accessed instead of being instantiated when the cache client is. More details are available in the linked PR comments.