Working on a python example plus fixing python grpc code (#6456)

Refactored python grpc code gen

Adds example server & client + fixes ci

Fixes generated code

Making sure we encode the reply string as utf8

Adds Readme details to clarify issue regarding encoding when python is sending/receiving

grpc/examples/python/greeter/server.py

@@ -0,0 +1,57 @@
+from concurrent import futures
+import sys		
+import argparse
+import grpc
+
+sys.path.insert(0, '../../../../../flatbuffers/python')
+
+import flatbuffers
+from models import HelloReply, HelloRequest, greeter_grpc_fb
+
+parser = argparse.ArgumentParser()
+parser.add_argument("port", help="server on port", default=3000)
+
+def build_reply(message):
+    builder = flatbuffers.Builder()		
+    ind = builder.CreateString(message)
+    HelloReply.HelloReplyStart(builder)
+    HelloReply.HelloReplyAddMessage(builder, ind)
+    root = HelloReply.HelloReplyEnd(builder)
+    builder.Finish(root)
+    return bytes(builder.Output())
+
+class GreeterServicer(greeter_grpc_fb.GreeterServicer):
+
+    def __init__(self):
+        self.greetings = ["Hi", "Hallo", "Ciao"]
+
+    def SayHello(self, request, context):
+        r = HelloRequest.HelloRequest().GetRootAs(request, 0)
+        reply = "Unknown"
+        if r.Name():
+            reply = r.Name()
+        return build_reply("welcome " + reply.decode('UTF-8'))
+
+    def SayManyHellos(self, request, context):
+        r = HelloRequest.HelloRequest().GetRootAs(request, 0)
+        reply = "Unknown"
+        if r.Name():
+            reply = r.Name()
+
+        for greeting in self.greetings:
+            print(type(reply))
+            yield build_reply(greeting + " " + reply.decode('UTF-8'))
+        
+
+def serve():
+    args = parser.parse_args()
+    server = grpc.server(futures.ThreadPoolExecutor(max_workers=10))
+    greeter_grpc_fb.add_GreeterServicer_to_server(
+        GreeterServicer(), server
+    )
+    server.add_insecure_port('[::]:' + args.port)
+    server.start()
+    server.wait_for_termination()
+
+if __name__ == '__main__':
+    serve()