[SPARK-52513] Add a streaming word count example with rolling event logs

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to add a streaming word count example, org.apache.spark.examples.streaming.HdfsWordCount.

In addition, this example will show a event log rolling feature which is enabled by default in Apache Spark 4.0.0.

spark.eventLog.enabled: "true"
spark.eventLog.dir: "s3a://spark-events/"
spark.eventLog.rolling.maxFileSize: "10m"

$ aws s3 --profile localstack ls s3://spark-events/ --recursive
2025-06-17 09:43:55          0 eventlog_v2_stream-word-count-0/
2025-06-17 09:43:55          0 eventlog_v2_stream-word-count-0/appstatus_stream-word-count-0.inprogress
2025-06-17 09:52:02    1957278 eventlog_v2_stream-word-count-0/events_1_stream-word-count-0.zstd

Why are the changes needed?

To show a streaming example with event log.

Does this PR introduce any user-facing change?

No behavior change because this is an example.

How was this patch tested?

Manual tests.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Thank you, @viirya . Merged to main.