request help: How to pass headers from plugin runner to upstream

### Issue description
We want to send custom headers from go plugin runner to the upstream. We have tried adding it to both request object and response object in custom plugin filter , but both are not working.
```
w http.ResponseWriter, r pkgHTTP.Request
r.Header().Set("x-team-info","abc")
w.Header().Add("x-team-info","abc)
```
Looking for a way to send headers from plugin runner filter to the upstream. We want to append new headers to the upstream.
### Environment

* APISIX Go Plugin Runner's version: master branch
* APISIX version: 2.11.0
* Go version: 1.6.2
* OS (cmd: `uname -a`): centos (linux amd 64)

cc: @Sudalaimuthu