[Aio] unary unary client call barebones implementation

[pfreixes]

We will be implementing the foundations of the Aio client by implementing the minimal stuff needed for sending a request and waiting for a response for a unary unary call.

After this is implemented we will be extending the barebones for having a full parity implementation compared to the synchronous client.

This issue can be scope within the Asyncio Client Implementation #19497 issue.

Acceptance Criteria

• Aio client can perform a unary unary call by sending a request and waiting for a response.

[pfreixes]

@gnossen could you add this into the gRPC easy project, please? Also, could you put it in-progress? I'm currently working on this.

[pfreixes]

IMO this can be moved to done /cc @lidizheng