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_generate_test.go

@@ -1,3 +1,3 @@
-package ivpn
+package updater
 
 //go:generate mockgen -destination=mocks_test.go -package $GOPACKAGE . Warner