accept custom grpc options in from_cloud function (#215)

Although the `DgraphClientStub` constructor accepts custom gRPC options
for connecting to on-prem/self-hosted Dgraph backends, the same cannot
be passed to the `from_cloud` function while connecting to Cloud
backends.

This PR allows passing of gRPC options like
`grpc.max_receive_message_length` to the `from_cloud` function.

Author: rarvikar

pydgraph/client_stub.py

@@ -105,14 +105,18 @@ def parse_host(cloud_endpoint):
     # client_stub = pydgraph.DgraphClientStub.from_cloud("cloud_endpoint", "api-key")
     # client = pydgraph.DgraphClient(client_stub)
     @staticmethod
-    def from_cloud(cloud_endpoint, api_key):
+    def from_cloud(cloud_endpoint, api_key, options=None):
         """Returns Dgraph Client stub for the Dgraph Cloud endpoint"""
         host = DgraphClientStub.parse_host(cloud_endpoint)
         creds = grpc.ssl_channel_credentials()
         call_credentials = grpc.metadata_call_credentials(
             lambda context, callback: callback((("authorization", api_key),), None))
         composite_credentials = grpc.composite_channel_credentials(
             creds, call_credentials)
+        if options==None:
+            options=[('grpc.enable_http_proxy', 0)]
+        else:
+            options.append(('grpc.enable_http_proxy', 0))
         client_stub = DgraphClientStub('{host}:{port}'.format(
-            host=host, port="443"), composite_credentials, options=(('grpc.enable_http_proxy', 0),))
+            host=host, port="443"), composite_credentials, options=options)
         return client_stub