[Spark runner] Support for ProcessingTime Timer for Spark Runner

[twosom]

Please add a meaningful description for your change here

fixes #33633

Problem Statement

The Spark Runner did not support timers due to the following technical challenges:

1. Timer Triggering Difficulties: UnboundedInOutIterator objects are periodically created and destroyed at each Spark Streaming batch interval, rendering the internal while (true) loop ineffective for timer management.

2. Sparse Key Issue: As a consequence of the above limitation, sparse keys lose their timer triggering capability once their corresponding UnboundedInOutIterator objects are destroyed, leaving no mechanism to fire pending timers.

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This PR resolves the above issues to enable Processing Time Timer functionality.

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[damccorm]

Thanks for putting this together

[damccorm]

Is this safe to do? I'd expect us to try to fire processing time timers (if unfired) before clearing them.

[twosom]

@damccorm

You're absolutely right! I've added TimerUtils.triggerExpiredTimers to fire expired processing time timers before clearing them. This ensures we don't lose any timer callbacks that should have been executed.

Please take a look at the implementation.

[twosom]

@damccorm Thanks for the feedback!
I'll fix it!

[github-actions]

Reminder, please take a look at this pr: @damccorm

[damccorm]

waiting on author

[twosom]

@damccorm
Sorry for the delay in addressing your feedback. I've now implemented the changes you suggested. Thank you for your patience!

[github-actions]

Reminder, please take a look at this pr: @damccorm

[damccorm]

Thanks!