feat: allow disabling response stream pre-fetch (#30)

Closes #25.

This PR adds the ability to disable automatically pre-fetching the first item of a stream returned by `*-Stream` gRPC callables. This hook will be used in PubSub to fix the [stalled stream issue](https://github.com/googleapis/python-pubsub/issues/93), while also not affecting Firestore, since the default behavior is preserved.

I realize the fix is far from ideal, but it's the least ugly among the approaches I tried, e.g. somehow passing the flag through `ResumableBidiRpc` (it's a messy rabbit hole). On the PubSub side monkeypatching the generated SubscriberClient will be needed, but it's a (relatively) clean one-liner:
```patch
diff --git google/cloud/pubsub_v1/gapic/subscriber_client.py google/cloud/pubsub_v1/gapic/subscriber_client.py
index e98a686..1d6c058 100644
--- google/cloud/pubsub_v1/gapic/subscriber_client.py
+++ google/cloud/pubsub_v1/gapic/subscriber_client.py
@@ -1169,6 +1169,8 @@ class SubscriberClient(object):
                 default_timeout=self._method_configs["StreamingPull"].timeout,
                 client_info=self._client_info,
             )
+            # TODO: explain this monkeypatch!
+            self.transport.streaming_pull._prefetch_first_result_ = False
 
         return self._inner_api_calls["streaming_pull"](
             requests, retry=retry, timeout=timeout, metadata=metadata
```

If/when we merge this, we should also release it, and then we can add `!= 1.17.0` to the `google-api-core` version pin in PubSub.

### PR checklist
- [x] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-api-core/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)

1 files changed, 13 insertions, 0 deletions

diff --git a/tests/unit/test_grpc_helpers.py b/tests/unit/test_grpc_helpers.py
index 1fec64f..ef84514 100644
--- a/tests/unit/test_grpc_helpers.py
+++ b/tests/unit/test_grpc_helpers.py
@@ -80,6 +80,19 @@ def test_wrap_stream_okay():
     assert responses == expected_responses
 
 
+def test_wrap_stream_prefetch_disabled():
+    responses = [1, 2, 3]
+    iter_responses = iter(responses)
+    callable_ = mock.Mock(spec=["__call__"], return_value=iter_responses)
+    callable_._prefetch_first_result_ = False
+
+    wrapped_callable = grpc_helpers._wrap_stream_errors(callable_)
+    wrapped_callable(1, 2, three="four")
+
+    assert list(iter_responses) == responses  # no items should have been pre-fetched
+    callable_.assert_called_once_with(1, 2, three="four")
+
+
 def test_wrap_stream_iterable_iterface():
     response_iter = mock.create_autospec(grpc.Call, instance=True)
     callable_ = mock.Mock(spec=["__call__"], return_value=response_iter)