[Go] Working on a go example plus fixing go grpc code (#6448)

Implemented server.go and half implemented client.go

Finishes implementation for greeter go example

Update grpc code for monster.fbs

Adds a couple of cpp methods

Adhere to gofmt standards

Adds a readme for issues with grpc

grpc/examples/README.md

@@ -0,0 +1,7 @@
+## Languages known issues
+
+### Go
+
+- Always requires the `content-type` of the payload to be set to `application/grpc+flatbuffers`
+
+example: `.SayHello(ctx, b, grpc.CallContentSubtype("flatbuffers"))`