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/provider.go

@@ -2,24 +2,27 @@ package privateinternetaccess
 
 import (
 	"math/rand"
+	"net/http"
 	"time"
 
 	"github.com/qdm12/gluetun/internal/constants/openvpn"
 	"github.com/qdm12/gluetun/internal/constants/providers"
 	"github.com/qdm12/gluetun/internal/provider/common"
+	"github.com/qdm12/gluetun/internal/provider/privateinternetaccess/updater"
 )
 
 type Provider struct {
 	storage    common.Storage
 	randSource rand.Source
 	timeNow    func() time.Time
+	common.Fetcher
 	// Port forwarding
 	portForwardPath string
 	authFilePath    string
 }
 
 func New(storage common.Storage, randSource rand.Source,
-	timeNow func() time.Time) *Provider {
+	timeNow func() time.Time, client *http.Client) *Provider {
 	const jsonPortForwardPath = "/gluetun/piaportforward.json"
 	return &Provider{
 		storage:         storage,
@@ -27,6 +30,7 @@ func New(storage common.Storage, randSource rand.Source,
 		randSource:      randSource,
 		portForwardPath: jsonPortForwardPath,
 		authFilePath:    openvpn.AuthConf,
+		Fetcher:         updater.New(client),
 	}
 }