[Feature/Documentation Request] - Add GCE support to regionInstanceGroupManagers

### Feature Request

Currently, only zonal MIGs are supported. I'm requesting a upgrade to the architecture in order to support regional instance group managers for future-proofing and also for:

- Increased quota
- Better handle zonal-failure

### Documentation Request

Meanwhile, please add to the documentation notes on how the plugin supports **only** zonal MIGs to avoid confusion. The following is an example suggestion:

>- Template configuration
> - (...)
> - `gcp_zone` : Zone of the existing zonal MIG.
> - `gcp_instance_group`: Name of the existing zonal MIG.

### Reference

- [regionInstanceGroupManagers](https://cloud.google.com/compute/docs/reference/rest/v1/regionInstanceGroupManagers)
- https://github.com/google/symphony-gcp/blob/6e12b54ea2deb255e5a6b0483473e90adb41693b/hf-provider/src/gce_provider/commands/request_machines.py#L56-L64

Start with hf-provider/src/gce_provider/commands/request_machines.py around lines 56-64 and review the regionInstanceGroupManagers reference. Define how regional instance group managers should be supported alongside the current zonal behavior, then update the documentation to state that only zonal MIGs are currently supported. Done means the regional behavior is implemented and the documentation no longer leaves the supported scope ambiguous.

Written by the indexing model from the issue text.