feat: add operations rest client to support long-running operations. (googleapis#311)

* feat: add operations rest client to support long-running operations.

* fix: address test coverage gaps in operations rest client.

* fix: removed stray print statement.

* fix: address lint, blacken, and mypy issues.

* fix: address pytype, more coverage issues

* fix: addressed additional pytype issues and one coverage line.

* fix: renamed OperationsRestClient to AbstractOperationsClient.

Co-authored-by: Kenneth Bandes <[email protected]>

Author: kbandes

google/api_core/operations_v1/__init__.py

@@ -14,7 +14,14 @@
 
 """Package for interacting with the google.longrunning.operations meta-API."""
 
+from google.api_core.operations_v1.abstract_operations_client import AbstractOperationsClient
 from google.api_core.operations_v1.operations_async_client import OperationsAsyncClient
 from google.api_core.operations_v1.operations_client import OperationsClient
+from google.api_core.operations_v1.transports.rest import OperationsRestTransport
 
-__all__ = ["OperationsAsyncClient", "OperationsClient"]
+__all__ = [
+    "AbstractOperationsClient",
+    "OperationsAsyncClient",
+    "OperationsClient",
+    "OperationsRestTransport"
+]