Add serviceProxyHealthy field to Service health check response in Antrea Proxy

[antoninbas]

There is a discrepancy between Antrea proxy and kube-proxy, as kube-proxy includes the field while Antrea Proxy does not. The field was added in kube-proxy about 2 years ago with kubernetes/kubernetes#111661. It is based on the observation that while there may be local Endpoints for a LoadBalancer Service (with externalTrafficPolicy: Local), it does not necessarily mean that the datapath rules (OVS flows, iptables rules...) realizing the Service have been installed successfully as the proxy itself may be experiencing some issues.

For reference, here is an API response for kube-proxy:

{
	"service": {
		"namespace": "default",
		"name": "nginx-test"
	},
	"localEndpoints": 1,
	"serviceProxyHealthy": true
}

And here is one for Antrea Proxy:

{
	"service": {
		"namespace": "default",
		"name": "nginx-test"
	},
	"localEndpoints": 1
}

We should add the missing field in Antrea Proxy.

It may be a good opportunity to audit the third_party/proxy code and look for additional discrepancies.

cc @hongliangl