api_listener: allow the api listener to configure settings for the synthetic connection

*Description*:
Right now the SyntheticConnection used inside the Api Listener is not configurable. For instance there is no way to change the return value of the [ssl()](https://github.com/envoyproxy/envoy/blob/9105aead0f5c89419bd2aca789b3738967c0baa1/source/server/api_listener_impl.h#L117) call. This can have implications in the requests that are created by the Api Listener. For instance, for the example above the value returned by that function dictates the value added for the `x-forwarded-proto` header [here](https://github.com/envoyproxy/envoy/blob/9105aead0f5c89419bd2aca789b3738967c0baa1/source/common/http/conn_manager_utility.cc#L134). Therefore, it might be valuable to allow the Api Listener to specify things about its Synthetic Connection.