IVPN server data update code and ISP filter (#578)

- Use IVPN's HTTP API instead of their .zip file
- Unit tests for API and GetServers
- Paves the way for Wireguard
- Update server information for IVPN
- Add `ISP` filter for IVPN

internal/updater/providers/ivpn/api.go

@@ -0,0 +1,66 @@
+package ivpn
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+var (
+	errBuildRequest          = errors.New("cannot build HTTP request")
+	errDoRequest             = errors.New("failed doing HTTP request")
+	errHTTPStatusCodeNotOK   = errors.New("HTTP status code not OK")
+	errUnmarshalResponseBody = errors.New("failed unmarshaling response body")
+	errCloseBody             = errors.New("failed closing HTTP body")
+)
+
+type apiData struct {
+	Servers []apiServer `json:"servers"`
+}
+
+type apiServer struct {
+	Hostnames apiHostnames `json:"hostnames"`
+	IsActive  bool         `json:"is_active"`
+	Country   string       `json:"country"`
+	City      string       `json:"city"`
+	ISP       string       `json:"isp"`
+}
+
+type apiHostnames struct {
+	OpenVPN string `json:"openvpn"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error) {
+	const url = "https://api.ivpn.net/v4/servers/stats"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, fmt.Errorf("%w: %s", errBuildRequest, err)
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, fmt.Errorf("%w: %s", errDoRequest, err)
+	}
+
+	if response.StatusCode != http.StatusOK {
+		_ = response.Body.Close()
+		return data, fmt.Errorf("%w: %d %s",
+			errHTTPStatusCodeNotOK, response.StatusCode, response.Status)
+	}
+
+	decoder := json.NewDecoder(response.Body)
+	if err := decoder.Decode(&data); err != nil {
+		_ = response.Body.Close()
+		return data, fmt.Errorf("%w: %s", errUnmarshalResponseBody, err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return data, fmt.Errorf("%w: %s", errCloseBody, err)
+	}
+
+	return data, nil
+}