Add event emitters using Jupyter events for GatewayClient and GatewayMappingKernelManager

[rajmusuku]

Problem

Jupyter doesn't have a way to capture enterprise gateway's client and kernel failures to be able to log them as events/metrics in the internal telemetry system. We would like to know

• How many gateway client failures took place in the last few days corresponding to number of users
• How many kernel failure took place.
• Error/exceptions from Enterprise gateway.

Proposed Solution

By leveraging jupyter_events, we can emit events for both GatewayClient and GatewayMappingKernelManger. On the receiver side, having a server extension that listens all the events and log them to internal telemetry system.

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[Zsailer]

Done in #1252.