Fix GCP resource labels for mixed-case owner

[kiryl-filatau]

Summary

GCP rejects resource labels if they contain uppercase letters (for example owner=Name_Surname). The owner field in _GetResourceDict is now passed through _SafeLabelKeyOrValue, consistent with how --metadata pairs are already handled. GCE metadata (like enable-oslogin=FALSE) is not changed.

---

What broke

Running on GCP:

```bash
./pkb.py --cloud=GCP --benchmarks=kubernetes_scale
```

Cluster create failed:

```text
ERROR: (gcloud.container.clusters.create) ResponseError: code=400, message=Invalid field 'resource_labels.value':
"Name_Surname". It must only contain lowercase letters ([a-z]), numeric characters ([0-9]), underscores (_) and dashes (-).
International characters are allowed.
```

owner comes from --owner / getpass.getuser() and was passed to gcloud --labels without lowercasing.

---

Fix

In perfkitbenchmarker/benchmark_spec.py (_GetResourceDict):

• owner is now wrapped in _SafeLabelKeyOrValue() — same as --metadata pairs

One unit test: testOwnerFromGetuserIsSanitized in benchmark_spec_test.py.

---

How was tested

• python3 -m pytest tests/benchmark_spec_test.py tests/providers/gcp/util_test.py -v
• ./pkb.py --cloud=GCP --benchmarks=kubernetes_scale — cluster create works, labels are lowercase