Cert Test: Simplify server-client stream handling

* Created EventCallbackStream in Python to handle gRPC server
  one-directional stream asynchronously
* Created EventAsserts in Python that uses EventCallbackStream
  to assert on incoming gRPC stream events
* Created bluetooth::grpc::GrpcEventQueue to handle gRPC server
  stream request asynchronously. This class blocks the calling
  gRPC thread whiling taking incoming events on a different
  thread and pass the event via a common::BlockingQueue
* Migrated all existing facade tests to use the new infrastructure
* Get rid of legacy definition of EventStream

Bug: 143373417
Test: SimpleHalTest, SimpleHciTest, SimpleL2capTest
Change-Id: I9d841fe2729e284450f8b031d9d1f094dd0c021b