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/wevpn/updater/cities.go

@@ -1,4 +1,4 @@
-package wevpn
+package updater
 
 // getAvailableCities get available cities as listed on the WeVPN website.
 func getAvailableCities() (cities []string) {