add httpclient roundtripper with proxy and user-agent

httpclient/httpclient_transport.go

@@ -0,0 +1,26 @@
+package httpclient
+
+import "net/http"
+
+type httpClientRoundTripper struct {
+	transport http.RoundTripper
+	config    *Config
+}
+
+func newHTTPClientTransport(config *Config) http.RoundTripper {
+	proxy := http.ProxyURL(config.Proxy)
+
+	transport := &http.Transport{
+		Proxy: proxy,
+	}
+
+	return &httpClientRoundTripper{
+		transport: transport,
+		config:    config,
+	}
+}
+
+func (h *httpClientRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	req.Header.Add("User-Agent", h.config.UserAgent)
+	return h.transport.RoundTrip(req)
+}