Include Plugins with GitSync

### Official Helm Chart version

1.18.0 (latest released)

### Apache Airflow version

3.1.5

### Kubernetes Version

v1.33.4

### Helm Chart configuration

GitSync syncs DAGs (and other configured paths) but does not sync the `$AIRFLOW_HOME/plugins` directory

Plugins placed in the repo are ignored unless they are manually baked into the image or mounted separately. https://airflow.apache.org/docs/apache-airflow/stable/administration-and-deployment/plugins.html

### Docker Image customizations

_No response_

### What happened

_No response_

### What you think should happen instead

_No response_

### How to reproduce

Deploy airflow with gitsync enabled, no special configuration.

### Anything else

_No response_

### Are you willing to submit PR?

- [ ] 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 official Helm chart's GitSync configuration and the linked Airflow plugins documentation. Trace how synced paths are made available to Airflow, then verify the chart can include the repository's plugins directory without special image or mount configuration; done means a deployment with GitSync loads those plugins.

Written by the indexing model from the issue text.