Add stringMethodOptionsExtension

This helper function can be used on service methods
to read options, which extend method options by a string.

e.g.

It is possible to read "Some string" for a custom

`my_method_option.bar` message option extension.

See:

https://developers.google.com/protocol-buffers/docs/proto#customoptions

on how to define a custom extension and referencing is by a fieldID.

```
service MyService {

  rpc MyMethod(RequestType) returns(ResponseType) {
    option (my_method_option.bar) = "Some string";
  }
}
```

If `my_method_option.bar` was defined for `fieldID: 50000`
one can reference it using protoc-gen-template like:

```
{{- stringMethodOptionsExtension 50000 .method }}
```

helpers/helpers.go

@@ -127,6 +127,7 @@ var ProtoHelpersFuncMap = template.FuncMap{
 	"trailingComment":              trailingComment,
 	"leadingDetachedComments":      leadingDetachedComments,
 	"stringFieldExtension":         stringFieldExtension,
+	"stringMethodOptionsExtension": stringMethodOptionsExtension,
 	"boolFieldExtension":           boolFieldExtension,
 	"isFieldMap":                   isFieldMap,
 	"fieldMapKeyType":              fieldMapKeyType,
@@ -234,6 +235,46 @@ func leadingDetachedComments(i interface{}) []string {
 	return loc.GetLeadingDetachedComments()
 }
 
+// stringMethodOptionsExtension extracts method options of a string type.
+// To define your own extensions see:
+// https://developers.google.com/protocol-buffers/docs/proto#customoptions
+// Typically the fieldID of private extensions should be in the range:
+// 50000-99999
+func stringMethodOptionsExtension(fieldID int32, f *descriptor.MethodDescriptorProto) string {
+	if f == nil {
+		return ""
+	}
+	if f.Options == nil {
+		return ""
+	}
+	var extendedType *descriptor.MethodOptions
+	var extensionType *string
+
+	eds := proto.RegisteredExtensions(f.Options)
+	if eds[fieldID] == nil {
+		ed := &proto.ExtensionDesc{
+			ExtendedType:  extendedType,
+			ExtensionType: extensionType,
+			Field:         fieldID,
+			Tag:           fmt.Sprintf("bytes,%d", fieldID),
+		}
+		proto.RegisterExtension(ed)
+		eds = proto.RegisteredExtensions(f.Options)
+	}
+
+	ext, err := proto.GetExtension(f.Options, eds[fieldID])
+	if err != nil {
+		return ""
+	}
+
+	str, ok := ext.(*string)
+	if !ok {
+		return ""
+	}
+
+	return *str
+}
+
 func stringFieldExtension(fieldID int32, f *descriptor.FieldDescriptorProto) string {
 	if f == nil {
 		return ""