[Bug] CustomExtractor cannot be cast to class org.apache.pulsar.functions.windowing.TimestampExtractor

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Version

2.10.2

### Minimal reproduce step

Described in https://github.com/tomaszmichalak/pulsar-sandbox.

### What did you expect to see?

No issue with custom TimestampExtractor implementation.

### What did you see instead?

java.lang.ClassCastException: class org.apache.pulsar.functions.watermark.SampleTimestampExtractor cannot be cast to class org.apache.pulsar.functions.windowing.TimestampExtractor (org.apache.pulsar.functions.watermark.SampleTimestampExtractor is in unnamed module of loader org.apache.pulsar.functions.utils.functioncache.FunctionClassLoaders$ParentFirstClassLoader @3872bc37; org.apache.pulsar.functions.windowing.TimestampExtractor is in unnamed module of loader java.net.URLClassLoader @3f102e87)
at org.apache.pulsar.functions.windowing.WindowFunctionExecutor.getTimeStampExtractor(WindowFunctionExecutor.java:167) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]
at org.apache.pulsar.functions.windowing.WindowFunctionExecutor.getWindowManager(WindowFunctionExecutor.java:111) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]
at org.apache.pulsar.functions.windowing.WindowFunctionExecutor.initialize(WindowFunctionExecutor.java:67) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]
at org.apache.pulsar.functions.instance.JavaInstance.initialize(JavaInstance.java:160) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]
at org.apache.pulsar.functions.instance.JavaInstanceRunnable.setup(JavaInstanceRunnable.java:234) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]
at org.apache.pulsar.functions.instance.JavaInstanceRunnable.run(JavaInstanceRunnable.java:260) ~[org.apache.pulsar-pulsar-functions-instance-2.10.2.jar:2.10.2]

### Anything else?

I try to find examples with watermark cases.

### Are you willing to submit a PR?

- [ ] I'm willing to submit a PR!

Start with the sandbox reproduction linked in the issue, then inspect WindowFunctionExecutor.getTimeStampExtractor at line 167 and its calls from getWindowManager and initialize. Confirm the class-loader interaction behind the ClassCastException and verify that a custom TimestampExtractor can initialize without this failure.

Written by the indexing model from the issue text.