bug: On applying any ext-plugin-post-resp, any header set by upstream service was not passed to downstream

### If we apply any ext-plugin-post-resp filter any header set by upstream service was not passed to downstream.
Also There is no provision for setting multiple headers as headers is made as private Map headers; instead it should be private Map> headers in PostResponse.java and PostRequest.java

### Environment

* apisix-runner-starter 0.4.0

### Minimal test code / Steps to reproduce the issue

1. Apply any developed filter on ext-plugin-post-resp
2. Set some header say cookie on upstream server
3. Check response on httpclient, upstream headers has been ommitted

### What's the actual result? (including assertion message & call stack if applicable)
Headers is not available on downstream

### What's the expected result?
Headers should be available on downstream

Attaching a fix for the same
[apisix.zip](https://github.com/apache/apisix-java-plugin-runner/files/12011915/apisix.zip)