Maintenance: refactor servers updater code

- Require at least 80% of number of servers now to pass
- Each provider is in its own package with a common structure
- Unzip package with unzipper interface
- Openvpn package with extraction and download functions

internal/updater/providers/pia/api.go

@@ -0,0 +1,74 @@
+package pia
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net"
+	"net/http"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
+)
+
+type apiData struct {
+	Regions []regionData `json:"regions"`
+}
+
+type regionData struct {
+	Name        string `json:"name"`
+	PortForward bool   `json:"port_forward"`
+	Offline     bool   `json:"offline"`
+	Servers     struct {
+		UDP []serverData `json:"ovpnudp"`
+		TCP []serverData `json:"ovpntcp"`
+	} `json:"servers"`
+}
+
+type serverData struct {
+	IP net.IP `json:"ip"`
+	CN string `json:"cn"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error) {
+	const url = "https://serverlist.piaservers.net/vpninfo/servers/v5"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return data, fmt.Errorf("%w: %s", ErrHTTPStatusCodeNotOK, response.Status)
+	}
+
+	b, err := ioutil.ReadAll(response.Body)
+	if err != nil {
+		return data, err
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return data, err
+	}
+
+	// remove key/signature at the bottom
+	i := bytes.IndexRune(b, '\n')
+	b = b[:i]
+
+	if err := json.Unmarshal(b, &data); err != nil {
+		return data, err
+	}
+
+	return data, nil
+}