Add type annotations to gcp_spanner.py.
Add type annotations to gcp_spanner.py.

PiperOrigin-RevId: 423361809

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker