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/ivpn/updater/mocks_test.go

@@ -2,7 +2,7 @@
 // Source: github.com/qdm12/gluetun/internal/provider/ivpn/updater (interfaces: Warner)
 
 // Package ivpn is a generated GoMock package.
-package ivpn
+package updater
 
 import (
 	reflect "reflect"