[Core] Update changelog for async generator (#3289)

# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
- [ ] **CHANGELOG is updated for new features, bug fixes or other
significant changes.**
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- [ ] **Create an issue and link to the pull request to get dedicated
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## General Guidelines and Best Practices
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- [ ] There are a small number of commits, each of which have an
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### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

src/promptflow-core/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### Features Added
 - Support modifying the promptflow logger format through environment variables, reach [here](https://microsoft.github.io/promptflow/how-to-guides/faq.html#set-logging-format) for more details.
+- Support async generator in flex flow.
 
 ## v1.10.0 (2024.04.26)