compute gateways per namespace only if `ScopeGatewayToNamespace` is enabled (#36956)
compute gateways per namespace only if `ScopeGatewayToNamespace` is enabled (#36956)

Signed-off-by: dddddai <be106325561fd98b51909b9dec34e06b3e983919@foxmail.com>
istio/istio,istio/istio,istio/istio,istio/istio,istio/istio