Add Decision Events Iterator implementation

[timl3136]

What changed?

• Added DecisionEventsIterator class as an async iterator for processing workflow decision events
• Decision Events Iterator worked with existing history_event_iterator for pagination.
• Implemented HistoryHelper interface and DecisionEvents dataclass for workflow replay functionality

Why?
We need support for iterating decision events for the task handler.

This implementation is based on the Java Cadence client's DecisionEventsIterator pattern:

• Java HistoryHelper: https://github.com/cadence-workflow/cadence-java-client/blob/0e1916eceee9bd73a9d41cca31c0d2afd413763e/src/main/java/com/uber/cadence/internal/replay/HistoryHelper.java

How did you test it?
Unit tests

Potential risks

Release notes

Documentation Changes