[BUG] Current Pulsar Client is not shutting down `EventLoopGroup` properly

[alexeykudinkin]

Describe the bug

While we're testing our integration with Pulsar Spark Connector, we found out that while shutting down PulsarClientImpl doesn't shutdown EventLoopGroup properly (which is seems to be addressed in most recent Pulsar Client 2.10.1)

To Reproduce Steps to reproduce the behavior:

1. Create PulsarClient (version from "pulsar-spark-connector")
2. Try shutting down Pulsar (everything will be wound down except eventLoopGroup, which will keep JVM from shutting down)

Expected behavior JVM to be shutdown after PulsarClient is successfully closed.

Screenshots NA

Additional context NA