Plugins: Refactor call resource API handling (#67234)

Moving call resource API stream handling within plugin management as a utility/wrapper. 

Closes #66889

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/services/pluginsintegration/clientmiddleware/resource_response_middleware.go

@@ -34,6 +34,10 @@ func (m *ResourceResponseMiddleware) CallResource(ctx context.Context, req *back
 	processedStreams := 0
 	wrappedSender := callResourceResponseSenderFunc(func(res *backend.CallResourceResponse) error {
 		if processedStreams == 0 {
+			if res.Headers == nil {
+				res.Headers = map[string][]string{}
+			}
+
 			proxyutil.SetProxyResponseHeaders(res.Headers)
 		}