Add manager classes for benchmark operations:

• BenchmarkOps: Synchronous manager with create(), from_id(), and list()
• AsyncBenchmarkOps: Async manager with the same interface

Changes:

• Add SDKBenchmarkCreateParams and SDKBenchmarkListParams types
• Wire BenchmarkOps to RunloopSDK.benchmark
• Wire AsyncBenchmarkOps to AsyncRunloopSDK.benchmark
• Update exports in init.py to include new classes

Tests:

• Add unit tests for BenchmarkOps
• Add unit tests for AsyncBenchmarkOps
• Update SDK initialization tests to verify benchmark attribute
• Update smoketests to use sdk_client.benchmark.* methods