Add changelog

Signed-off-by: gaobinlong <[email protected]>

CHANGELOG.md

@@ -10,6 +10,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - fix: make sure $schema always added to LLM generated vega json object([#252](https://github.com/opensearch-project/dashboards-assistant/pull/252))
 - feat: added a new visualization type visualization-nlq to support creating visualization from natural language([#264](https://github.com/opensearch-project/dashboards-assistant/pull/264))
 - feat: exposed an API to check if a give agent config name has configured with agent id([#307](https://github.com/opensearch-project/dashboards-assistant/pull/307))
+- fix: make dataSourceId parameter optional for agent config_exist API ([#311](https://github.com/opensearch-project/dashboards-assistant/pull/311))
 
 ### 📈 Features/Enhancements