Feature Request: Proxy configurations for gcpserviceaccount auth

### Checklist

- [x] I did not find a related open enhancement request.
- [x] I understand that enhancement requests filed in the GitHub repository are by default low priority.
- [x] If this request is time-sensitive, I have submitted a corresponding issue with [GCP support](https://cloud.google.com/support-hub).

### Describe the feature

The customer is requesting native support for proxy configurations in Config Sync that differ from the current limitations. Specifically, they need proxy settings that work with the gcpserviceaccount authentication type.

### Importance

_No response_