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/privateinternetaccess/updater/servers.go

@@ -1,6 +1,6 @@
-// Package pia contains code to obtain the server information
+// Package updater contains code to obtain the server information
 // for the Private Internet Access provider.
-package privateinternetaccess
+package updater
 
 import (
 	"context"