*: add health handler for grpcproxy self

[tangcong]

When grpcproxy hangs or is overloaded, the watch client will not receive any change event and cannot detect the proxy failure. We need a health check to remove the abnormal proxy in time.

[tangcong]

grpcproxy may have a different certificate than etcd, now proxy/health works well when the user enables auto-tls, but i am not sure whether we should add the specified client certificate flags for the health check. proxy cert(grpcProxyListenCert) will be used as a client certificate in health check scenarios. the user-defined proxy certificate may cause the health check to time out. @gyuho

[gyuho]

proxy cert(grpcProxyListenCert) will be used as a client certificate in health check scenarios

Think this is good for now.

[tangcong]

proxy cert(grpcProxyListenCert) will be used as a client certificate in health check scenarios

Think this is good for now.

ok, thanks. I will follow the feedback from the community. If anyone meets the problem, one of the solutions is that cert usages can include client auth.

            "server": {
                "expiry": "2540400h",
                "usages": [
                    "signing",
                    "key encipherment",
                    "server auth",
		    "client auth"
                ]
            },