[SPARK-45497][K8S] Add a symbolic link file spark-examples.jar in K8s Docker images

### What changes were proposed in this pull request?

This PR aims to add a symbolic link file, `spark-examples.jar`, in the example jar directory.

```
$ docker run -it --rm spark:latest ls -al /opt/spark/examples/jars  | tail -n6
total 1620
drwxr-xr-x 1 root root    4096 Oct 11 04:37 .
drwxr-xr-x 1 root root    4096 Sep  9 02:08 ..
-rw-r--r-- 1 root root   78803 Sep  9 02:08 scopt_2.12-3.7.1.jar
-rw-r--r-- 1 root root 1564255 Sep  9 02:08 spark-examples_2.12-3.5.0.jar
lrwxrwxrwx 1 root root      29 Oct 11 04:37 spark-examples.jar -> spark-examples_2.12-3.5.0.jar
```

### Why are the changes needed?

Like PySpark example (`pi.py`), we can submit the examples without considering the version numbers which was painful before.
```
bin/spark-submit \
--master k8s://$K8S_MASTER \
--deploy-mode cluster \
...
--class org.apache.spark.examples.SparkPi \
local:///opt/spark/examples/jars/spark-examples.jar 10000
```

The following is the driver pod log.
```
+ exec /usr/bin/tini -s -- /opt/spark/bin/spark-submit ...
--deploy-mode client
--properties-file /opt/spark/conf/spark.properties
--class org.apache.spark.examples.SparkPi
local:///opt/spark/examples/jars/spark-examples.jar 10000
Files  local:///opt/spark/examples/jars/spark-examples.jar from /opt/spark/examples/jars/spark-examples.jar to /opt/spark/work-dir/./spark-examples.jar
```

### Does this PR introduce _any_ user-facing change?

No, this is an additional file.

### How was this patch tested?

Manually build the docker image and do `ls`.

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

No.

Closes apache#43324 from dongjoon-hyun/SPARK-45497.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/Dockerfile

@@ -59,6 +59,7 @@ COPY sbin /opt/spark/sbin
 COPY kubernetes/dockerfiles/spark/entrypoint.sh /opt/
 COPY kubernetes/dockerfiles/spark/decom.sh /opt/
 COPY examples /opt/spark/examples
+RUN ln -s $(basename $(ls /opt/spark/examples/jars/spark-examples_*.jar)) /opt/spark/examples/jars/spark-examples.jar
 COPY kubernetes/tests /opt/spark/tests
 COPY data /opt/spark/data