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/unzip.go

@@ -0,0 +1,22 @@
+// Package unzip defines the Unzipper which fetches and extract a zip file
+// containing multiple files.
+package unzip
+
+import (
+	"context"
+	"net/http"
+)
+
+type Unzipper interface {
+	FetchAndExtract(ctx context.Context, url string) (contents map[string][]byte, err error)
+}
+
+type unzipper struct {
+	client *http.Client
+}
+
+func New(client *http.Client) Unzipper {
+	return &unzipper{
+		client: client,
+	}
+}