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/unzip/fetch.go

@@ -0,0 +1,52 @@
+package unzip
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+)
+
+var (
+	ErrHTTPStatusCodeNotOK = errors.New("HTTP status code not OK")
+)
+
+func (u *unzipper) FetchAndExtract(ctx context.Context, url string) (
+	contents map[string][]byte, err error) {
+	contents = make(map[string][]byte)
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := u.client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%w: %s for %s", ErrHTTPStatusCodeNotOK, response.Status, url)
+	}
+
+	b, err := ioutil.ReadAll(response.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	newContents, err := zipExtractAll(b)
+	if err != nil {
+		return nil, err
+	}
+	for fileName, content := range newContents {
+		contents[fileName] = content
+	}
+
+	return contents, nil
+}