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/updater/html/fetch.go

@@ -0,0 +1,43 @@
+package html
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+
+	"golang.org/x/net/html"
+)
+
+var ErrHTTPStatusCodeNotOK = errors.New("HTTP status code is not OK")
+
+func Fetch(ctx context.Context, client *http.Client, url string) (
+	rootNode *html.Node, err error) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("creating HTTP request: %w", err)
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%w: %d %s", ErrHTTPStatusCodeNotOK,
+			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
+}