Adding interface methods for object store bucket management.
Adding interface methods for object store bucket management.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=296445634

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