Major goal: improve sampler interface to support batching/async

[Strilanc]

[balopat]

Batching is added.
Async is still outstanding.

[maffoo]

Updates:

• We added duet as a dependency of cirq-core and have stubbed out async versions of run, run_sweep, and run_batch on cirq.Sampler (Update Sampler.run_async signature to match Sampler.run #5036).
• We updated the gapic generated code for the quantum engine API which includes async support with asyncio (Import new gapic generated code for quantum engine API #5139).
• We have a prototype of adding async support to various Engine* classes (Add async support in EngineClient, EngineSampler, etc. #5219).

[dstrain115]

I think this goal has been achieved given maffoo's comments. We have also added a run_batch interface to engine as well. If not, we should clarify what specific things should still be added to accomplish this goal.

[maffoo]

There are some additional things we want to add (proper async interface to Quantum Engine, "dynamic" batching that adapts based on request size, etc), but I think the sampler interface itself is in reasonable shape, so we can probably close this and address further improvements in separate issues/PRs.