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 }}
```
6765ff04eb