Support for Golang GRPC (Experimental) (#4082)

* support for grpc golang

* refactored grpc go generator

* added grpc-go test and refactored

* refactored idl_gen_grpc.cpp

* fixed grpc generate method name

* refactored flatc and fixed line length issue

* added codec to go lib and fixed formatting issues

* fixed spacing issues

go/grpc.go

@@ -0,0 +1,23 @@
+package flatbuffers
+
+// FlatbuffersCodec implements gRPC-go Codec which is used to encode and decode messages.
+var Codec string = "flatbuffers"
+
+type FlatbuffersCodec struct{}
+
+func (FlatbuffersCodec) Marshal(v interface{}) ([]byte, error) {
+	return v.(*Builder).FinishedBytes(), nil
+}
+
+func (FlatbuffersCodec) Unmarshal(data []byte, v interface{}) error {
+	v.(flatbuffersInit).Init(data, GetUOffsetT(data))
+	return nil
+}
+
+func (FlatbuffersCodec) String() string {
+	return Codec
+}
+
+type flatbuffersInit interface {
+	Init(data []byte, i UOffsetT)
+}