Support Merged Gateways in Gateway Namespace Mode

*Description*:
>Describe the desired behaviour, what scenario it enables and how it
would be used.

Currently, it is not possible to run these modes together.
However, this case can also be desired by the end users
We should track the working solution under this issue

[optional *Relevant Links*:]
>Any extra documentation required to understand the issue.
- https://gateway.envoyproxy.io/docs/tasks/operations/gateway-namespace-mode/
- https://github.com/envoyproxy/gateway/issues/7618