feat(pubsub): Implement Ping/Pong Mechanism to Improve Connection Reliability

[0xIchigo]

Problem

Both the blocking and nonblocking PubsubClient clients currently lack a mechanism to detect when the WebSocket connection becomes unresponsive or stale. Without periodic health checks, the clients may not realize that the server is no longer responding. This can lead to missed messages and unreliable subscriptions. This can affect applications relying on WebSockets to provide real-time data streams

Summary of Changes

This PR aims to implement a ping/pong mechanism in both the blocking and nonblocking PubsubClient clients to improve connection reliability

Changes

• Introduced a periodic Ping message sent to the server using tokio::time::interval
• Added handling for incoming Pong messages and reset the unmatched_pings counter upon receiving any message
• Configured the client to close the connection gracefully if the unmatched_pings counter exceeds DEFAULT_MAX_FAILED_PINGS
• Ensured the changes did not significantly alter the existing code structure. This meant hardcoding DEFAULT_PING_DURATION_SECONDS and DEFAULT_MAX_FAILED_PINGS, as making them configurable would've meant changing the existing subscription parameters resulting in a breaking change. However, these params could easily be changed in the future to become configurable—the importance here is introducing reliable health checks to WebSocket connections