GCP: Custom subnet names were incorrectly using the VPC name

[jimmycgz]

## Issue Description: 
At GCP, when custom subnet names provided in the benchmark argument, they got incorrectly copied from the network name

## Suggested Changes:
- Modify GceNetwork class to properly handle subnet names
- Ensure network name and subnet names are managed separately
- Fix references to subnet resources to use the correct network name

## Test command to reproduce and verify this fix

source ../../venv/bin/activate
cd $HOME/projects/PerfKitBenchmarker
export LOG_FOLDER=../pkb-logs
export MACHINE_TYPE=c4d-standard-4

python3 ./pkb.py \
  --benchmarks=nginx \
  --temp_dir=$LOG_FOLDER \
  --cloud=GCP \
  --project=your-gcp-proj1 \
  --zone=us-central1-a \
  --machine_type=$MACHINE_TYPE \
  --run_uri=standalone \
  --nginx_server_machine_type=$MACHINE_TYPE \
  --nginx_upstream_server_machine_type=$MACHINE_TYPE \
  --nginx_client_machine_type=$MACHINE_TYPE \
  --nginx_content_size=1024 \
  --nginx_load_configs="32:300:32:1000,512:300:512:1000" \
  --gce_network_name=your-custom-vpc \
  --gce_subnet_name=your-custom-subnet \
  --owner=owner-name \
  --accept_licenses

## Error
2025-05-05 16:42:30,314 standalone Thread-2 (_ExecuteBackgroundThreadTasks) nginx(1/1) gce_network.py:601 INFO     Ran: {gcloud compute firewall-rules create perfkit-firewall-standalone-22-22 --allow tcp:22-22,udp:22-22 --format json --network my-subnet --project my-project1 --quiet}
ReturnCode:1
STDOUT: 
STDERR: Creating firewall...
failed.
ERROR: (gcloud.compute.firewall-rules.create) Could not fetch resource:
- The resource 'projects/my-project1/global/networks/my-subnet' was not found

Pull Request with suggested fix: #5725

[pmkc]

Hmm this does seem like an issue, but I recommend never passing --gce_network_name=your-custom-vpc.
subnet names are unique and passing both network and subnet is never required.