fix(protonvpn): authenticated servers data updating (#2878)

- `-proton-username` flag for cli update
- `-proton-password` flag for cli update
- `UPDATER_PROTONVPN_USERNAME` option for periodic updates
- `UPDATER_PROTONVPN_PASSWORD` option for periodic updates

internal/provider/protonvpn/updater/servers.go

@@ -13,9 +13,26 @@ import (
 func (u *Updater) FetchServers(ctx context.Context, minServers int) (
 	servers []models.Server, err error,
 ) {
-	data, err := fetchAPI(ctx, u.client)
+	switch {
+	case u.username == "":
+		return nil, fmt.Errorf("%w: username is empty", common.ErrCredentialsMissing)
+	case u.password == "":
+		return nil, fmt.Errorf("%w: password is empty", common.ErrCredentialsMissing)
+	}
+
+	apiClient, err := newAPIClient(ctx, u.client)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("creating API client: %w", err)
+	}
+
+	cookie, err := apiClient.authenticate(ctx, u.username, u.password)
+	if err != nil {
+		return nil, fmt.Errorf("authentifying with Proton: %w", err)
+	}
+
+	data, err := apiClient.fetchServers(ctx, cookie)
+	if err != nil {
+		return nil, fmt.Errorf("fetching logical servers: %w", err)
 	}
 
 	countryCodes := constants.CountryCodes()