[AsyncIO] Overview of gRPC AsyncIO integration

[lidizheng]

AsyncIO introduces async/await keywords. In order to plumb through the asynchronous-ness of functions, the library has to expose async functions on its surface. So, while integrating gRPC AsyncIO, we might need to instrument following classes in order to make gRPC AsyncIO work.

Also, since python-api-core supports all Python versions, we can't use any async/await in existing modules. If needed, the import of new AsyncIO modules will be protected by explicit version check.

A all-in-one draft PR can be found: #22.

• google.api_core.async_future
• google.api_core.gapic_v1.config_async
• google.api_core.gapic_v1.method_async
• google.api_core.operations_v1.operations_async_client
• google.api_core.grpc_helpers_async
• google.api_core.operation_async
• google.api_core.retry_async

Unit tests included for all new modules under tests/asyncio.

For each module, I will create separate issues and PRs later.

[lidizheng]

With all components released, this issue should be closed.