Add optional flag to run redis memtier benchmark with custom memtier server selection.
Add optional flag to run redis memtier benchmark with custom memtier server selection.

PiperOrigin-RevId: 475569126

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