fix: Fix Header Duplication (#1568)

When using connectrpc to perform calls, duplicate headers are introduced
if the response object is returned directly by the runner or the
controller. This causes bugs when FTL is deployed to an environment that
does not work when there are duplicate headers, such as an environment
where envoy is used to facilitate traffic.

The fix for this is to instantiate a new response object to return from
the Call functions.

For more information on the envoy bug, see the issue linked below. 
envoyproxy/envoy#9019

The fix for this took place after reaching out to the Buf team on slack
https://bufbuild.slack.com/archives/CRZ680FUH/p1716344335783839

backend/controller/controller.go

@@ -793,7 +793,8 @@ func (s *Service) callWithRequest(
 	ctx = rpc.WithVerbs(ctx, append(callers, verbRef))
 	headers.AddCaller(req.Header(), schema.RefFromProto(req.Msg.Verb))
 
-	resp, err := client.verb.Call(ctx, req)
+	response, err := client.verb.Call(ctx, req)
+	resp := connect.NewResponse(response.Msg)
 	var maybeResponse optional.Option[*ftlv1.CallResponse]
 	if resp != nil {
 		maybeResponse = optional.Some(resp.Msg)

backend/runner/runner.go

@@ -196,7 +196,8 @@ func (s *Service) Call(ctx context.Context, req *connect.Request[ftlv1.CallReque
 	if !ok {
 		return nil, connect.NewError(connect.CodeUnavailable, errors.New("no deployment"))
 	}
-	return deployment.plugin.Client.Call(ctx, req)
+	response, err := deployment.plugin.Client.Call(ctx, req)
+	return connect.NewResponse(response.Msg), err
 }
 
 func (s *Service) Reserve(ctx context.Context, c *connect.Request[ftlv1.ReserveRequest]) (*connect.Response[ftlv1.ReserveResponse], error) {