help request: how to rewrite request header in java runner-plugin development

### Description

one request sample:
fore-end request with header [a]-> apisix request with header [b]-> back-end
how i transfer header [a] to [b] in java runner-plugin development ?

### Environment

- APISIX version (apisix version :2.15.0, java-plugin version:0.3.0 ):
- Operating system (run `uname -a`):
- OpenResty / Nginx version (run `openresty -V` or `nginx -V`):
- etcd version, if relevant (run `curl http://127.0.0.1:9090/v1/server_info`):
- APISIX Dashboard version, if relevant:
- Plugin runner version, for issues related to plugin runners:
- LuaRocks version, for installation issues (run `luarocks --version`):