Add boolean option to Helm Chart for splitting api-server

### Description Implement a top-level boolean flag for handling the splitting of api-server into core & execution per https://airflow.apache.org/docs/apache-airflow/stable/administration-and-deployment/web-stack.html#separating-api-servers ### Use case/motivation The motivation for this is to harden the Airflow deployment. Currently if the UI gets overwhelmed through bad queries (n+1) or just too many users / large dags the api-servers can stop responding for both the UI (not good) & the workers (really bad). See this issue for more info https://github.com/apache/airflow/issues/56635#issuecomment-3541585677 ### Related issues https://github.com/apache/airflow/issues/56635#issuecomment-3541585677 ### Are you willing to submit a PR? - [x] Yes I am willing to submit a PR! ### Code of Conduct - [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)