Implement simple .connect and close for client

## What changes are proposed in this pull request?
A simple implementation of the `connect` and `close` methods for the new client. There are a number of follow-on tickets:
- Implement support for the SSL args (https://databricks.atlassian.net/browse/SC-77240)
- Implement support for no-explicit open execution (https://databricks.atlassian.net/browse/SC-77289)
- Implement support for the client explicitly setting the session id (https://databricks.atlassian.net/browse/SC-77290)

But I plan to return to those after I have the basic execute & fetch functionality.

Also fixing the flakiness we've seen in the driver local tests and re-enabling them. It seems as though the command is sometimes still returned in the running state (and was also sometimes timing out).
## How is this tested?
- Added a new DriverLocalTest that checks we can make a connection without everything blowing up
- New unit tests
- Running the driver local test suites 5 times to ensure that the flakiness was fixed

Author: Niall Egan <niall.egan@databricks.com>
Author: Ubuntu <ubuntu@ip-10-110-20-37.us-west-2.compute.internal>

#97384 is resolved by NiallEgan/connection_method_for_client.

Author: Niall Egan

cmdexec/clients/python/command_exec_client.py

@@ -0,0 +1,71 @@
+import logging
+from typing import Dict, Tuple, List
+
+import grpc
+from google.protobuf import message
+
+import cmdexec.clients.python.sql_command_service_pb2 as command_pb2
+from cmdexec.clients.python.sql_command_service_pb2_grpc import SqlCommandServiceStub
+from cmdexec.clients.python.errors import OperationalError, InterfaceError
+
+logger = logging.getLogger(__name__)
+
+
+def connect(**kwargs):
+    return Connection(**kwargs)
+
+
+class Connection:
+    def __init__(self, **kwargs):
+        try:
+            self.host = kwargs["HOST"]
+            self.port = kwargs["PORT"]
+        except KeyError:
+            raise InterfaceError("Please include arguments HOST and PORT in kwargs for Connection")
+
+        self._base_client = CmdExecBaseHttpClient(self.host, self.port, kwargs.get("metadata", []))
+        open_session_request = command_pb2.OpenSessionRequest(
+            configuration={},
+            client_session_id=None,
+            session_info_fields=None,
+        )
+
+        try:
+            resp = self._base_client.make_request(self._base_client.stub.OpenSession,
+                                                  open_session_request)
+            self.session_id = resp.id
+            logger.info("Successfully opened session " + str(self.session_id.hex()))
+        except grpc.RpcError as error:
+            raise OperationalError("Error during database connection", error)
+
+    def cursor(self):
+        pass
+
+    def close(self):
+        try:
+            close_session_request = command_pb2.CloseSessionRequest(id=self.session_id)
+            self._base_client.make_request(self._base_client.stub.CloseSession,
+                                           close_session_request)
+        except grpc.RpcError as error:
+            raise OperationalError("Error during database connection close", error)
+
+
+class CmdExecBaseHttpClient:
+    """
+    A thin wrapper around a gRPC channel that takes cares of headers etc.
+    """
+
+    def __init__(self, host: str, port: int, http_headers: List[Tuple[str, str]]):
+        self.host_url = host + ":" + str(port)
+        self.http_headers = [(k.lower(), v) for (k, v) in http_headers]
+        self.channel = grpc.insecure_channel(self.host_url)
+        self.stub = SqlCommandServiceStub(self.channel)
+
+    def make_request(self, method, request):
+        try:
+            response = method(request, metadata=self.http_headers)
+            logger.info("Received message: %s", response)
+            return response
+        except grpc.RpcError as error:
+            logger.error("Received error during gRPC request: %s", error)
+            raise error

cmdexec/clients/python/errors.py

@@ -0,0 +1,38 @@
+class Error(Exception):
+    pass
+
+
+class Warning(Exception):
+    pass
+
+
+class InterfaceError(Error):
+    pass
+
+
+class DatabaseError(Error):
+    pass
+
+
+class InternalError(DatabaseError):
+    pass
+
+
+class OperationalError(DatabaseError):
+    pass
+
+
+class ProgrammingError(DatabaseError):
+    pass
+
+
+class IntegrityError(DatabaseError):
+    pass
+
+
+class DataError(DatabaseError):
+    pass
+
+
+class NotSupportedError(DatabaseError):
+    pass