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)

Start with the Airflow Helm Chart configuration and the linked documentation on separating API servers. Trace how the chart currently deploys the api-server and determine where a top-level boolean option would control separate core and execution services. Done means the option renders the intended split while preserving the existing default behavior.

Written by the indexing model from the issue text.