support Egress gateway and enhance observability of outbound traffic.

[LiZhenCheng9527]

What would you like to be added:

• support REGISTER_ONLY and ALLOW_ANY mode of egress gateway.
• Add result of connection failed in accesslog. Let the user know if it's a normal failure due to authz or an abnormal failure.

Why is this needed:
"Network perimeter security" is a key concern for any enterprise operations team. Network outbound traffic also needs to be secured.
But Kmesh doesn't do anything about outbound traffic right now. While there is no handling of this in ambient mesh either, the handling of outbound traffic is controlled in istio sidecar mode using REGISTER_ONLY and ALLOW_ANY.
Once the outbound traffic has been controlled, we need to add the reason for the connection failure in the accesslog. So that the user knows whether the link failed because of an expected or unexpected result.