feat: add VPNsecure.me support (#848)

- `OPENVPN_ENCRYPTED_KEY` environment variable 
- `OPENVPN_ENCRYPTED_KEY_SECRETFILE` environment variable 
- `OPENVPN_KEY_PASSPHRASE` environment variable 
- `OPENVPN_KEY_PASSPHRASE_SECRETFILE` environment variable 
- `PREMIUM_ONLY` environment variable
- OpenVPN user and password not required for vpnsecure provider

internal/provider/slickvpn/updater/website.go

@@ -13,7 +13,8 @@ import (
 
 func fetchServers(ctx context.Context, client *http.Client) (
 	hostToData map[string]serverData, err error) {
-	rootNode, err := fetchHTML(ctx, client)
+	const url = "https://www.slickvpn.com/locations/"
+	rootNode, err := htmlutils.Fetch(ctx, client, url)
 	if err != nil {
 		return nil, fmt.Errorf("fetching HTML code: %w", err)
 	}
@@ -26,39 +27,6 @@ func fetchServers(ctx context.Context, client *http.Client) (
 	return hostToData, nil
 }
 
-var ErrHTTPStatusCode = errors.New("HTTP status code is not OK")
-
-func fetchHTML(ctx context.Context, client *http.Client) (rootNode *html.Node, err error) {
-	const url = "https://www.slickvpn.com/locations/"
-	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
-	if err != nil {
-		return nil, err
-	}
-
-	response, err := client.Do(request)
-	if err != nil {
-		return nil, err
-	}
-
-	if response.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("%w: %d %s",
-			ErrHTTPStatusCode, response.StatusCode, response.Status)
-	}
-
-	rootNode, err = html.Parse(response.Body)
-	if err != nil {
-		_ = response.Body.Close()
-		return nil, fmt.Errorf("parsing HTML code: %w", err)
-	}
-
-	err = response.Body.Close()
-	if err != nil {
-		return nil, fmt.Errorf("closing response body: %w", err)
-	}
-
-	return rootNode, nil
-}
-
 type serverData struct {
 	ovpnURL string
 	country string