Maint: internal/storage rework

- No more global variables
- Inject merged servers to configuration package
- Fix #566: configuration parsing to use persisted servers.json
- Move server data files from `internal/constants` to `internal/storage`

internal/storage/hardcoded.go

@@ -0,0 +1,21 @@
+package storage
+
+import (
+	"embed"
+	"encoding/json"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+//go:embed servers.json
+var allServersEmbedFS embed.FS //nolint:gochecknoglobals
+
+func parseHardcodedServers() (allServers models.AllServers, err error) {
+	f, err := allServersEmbedFS.Open("servers.json")
+	if err != nil {
+		return allServers, err
+	}
+	decoder := json.NewDecoder(f)
+	err = decoder.Decode(&allServers)
+	return allServers, err
+}