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/expressvpn/connection_test.go

@@ -87,7 +87,9 @@ func Test_Provider_GetConnection(t *testing.T) {
 				Return(testCase.filteredServers, testCase.storageErr)
 			randSource := rand.NewSource(0)
 
-			provider := New(storage, randSource)
+			unzipper := (common.Unzipper)(nil)
+			warner := (common.Warner)(nil)
+			provider := New(storage, randSource, unzipper, warner)
 
 			if testCase.panicMessage != "" {
 				assert.PanicsWithValue(t, testCase.panicMessage, func() {