chore(updater): incorporate FetchServers method in Provider interface

- Each provider interface can now fetch updated servers data
- Rename each provider updater subpackage name to `updater`
- Updater constructor does not take a settings struct
- Updater update method takes in a slice of provider strings

internal/provider/protonvpn/updater/api.go

@@ -1,4 +1,4 @@
-package protonvpn
+package updater
 
 import (
 	"context"