Add gRPC server example with file download

TestGrpc/Protos/greet.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+option csharp_namespace = "TestGrpc";
+
+package greet;
+
+service Greeter {
+  rpc DownloadFile (FileRequest) returns (stream ChunkMsg);
+}
+
+message FileRequest {
+  string FilePath = 1;
+}
+
+message ChunkMsg {
+  string FileName = 1;
+  int64 FileSize = 2;
+  bytes Chunk = 3;
+}