Add cirq.Sampler.run_sweep_async (#1878)
Add cirq.Sampler.run_sweep_async (#1878)

- Streamline type annotations to only use 'cirq.'
- Extracted run_on_thread_async
quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq