CheckConnectivity determines whether the Shadowsocks proxy can relay TCP and UDP traffic under the current network.

CheckTCPConnectivityWithHTTP determines whether the proxy is reachable over TCP and validates the client's authentication credentials by performing an HTTP HEAD request to `targetURL`, which must be of the form: http://[host](:[port])(/[path]).

CheckUDPConnectivityWithDNS determines whether the Shadowsocks proxy represented by `client` and the network support UDP traffic by issuing a DNS query though a resolver at `resolverAddr`.