Support for Helm type syncs via config/rootSync in the Google terraform provider

### 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

Context: As a GKE Platform Engineer, the customer utilizes Terraform to manage Config Sync via Helm. They require the addition of Helm support to the sourceType field within the config-management block (to match existing Git and OCI functionality). This will enable them to deploy Custom Compute Classes using Helm charts as the single source of truth, directly managed by Terraform, without needing to maintain an intermediate Git repository solely for bootstrapping purposes.

### Importance

Impact: Without this functionality, the customer is forced to implement a "double-hop" solution. This involves Terraform configuring Config Sync to sync from a Git repository, which then contains a RootSync manifest that redirects to the Helm chart. This workaround doubles the complexity of their deployment process and prevents them from managing the deployment completely "out of band" via Google automation.

Blocker References: [b/478792323] [b/264556370] [b/270753674] [https://github.com/hashicorp/terraform-provider-google/issues/13379]