### What changes were proposed in this pull request?
Adding a dedicated boss event loop group to the Netty pipeline in the External Shuffle Service to avoid the delay in channel registration.
```
EventLoopGroup bossGroup = NettyUtils.createEventLoop(ioMode, 1,
conf.getModuleName() + "-boss");
EventLoopGroup workerGroup = NettyUtils.createEventLoop(ioMode, conf.serverThreads(),
conf.getModuleName() + "-server");
bootstrap = new ServerBootstrap()
.group(bossGroup, workerGroup)
.channel(NettyUtils.getServerChannelClass(ioMode))
.option(ChannelOption.ALLOCATOR, allocator)
```
### Why are the changes needed?
We have been seeing a large number of SASL authentication (RPC requests) timing out with the external shuffle service.
```
java.lang.RuntimeException: java.util.concurrent.TimeoutException: Timeout waiting for task.
at org.spark-project.guava.base.Throwables.propagate(Throwables.java:160)
at org.apache.spark.network.client.TransportClient.sendRpcSync(TransportClient.java:278)
at org.apache.spark.network.sasl.SaslClientBootstrap.doBootstrap(SaslClientBootstrap.java:80)
at org.apache.spark.network.client.TransportClientFactory.createClient(TransportClientFactory.java:228)
at org.apache.spark.network.client.TransportClientFactory.createUnmanagedClient(TransportClientFactory.java:181)
at org.apache.spark.network.shuffle.ExternalShuffleClient.registerWithShuffleServer(ExternalShuffleClient.java:141)
at org.apache.spark.storage.BlockManager$$anonfun$registerWithExternalShuffleServer$1.apply$mcVI$sp(BlockManager.scala:218)
```
The investigation that we have done is described here:
https://github.com/netty/netty/issues/9890
After adding `LoggingHandler` to the netty pipeline, we saw that the registration of the channel was getting delay which is because the worker threads are busy with the existing channels.
### Does this PR introduce any user-facing change?
No
### How was this patch tested?
We have tested the patch on our clusters and with a stress testing tool. After this change, we didn't see any SASL requests timing out. Existing unit tests pass.
Closes#27240 from otterc/SPARK-30512.
Authored-by: Chandni Singh <chsingh@linkedin.com>
Signed-off-by: Thomas Graves <tgraves@apache.org>
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