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/common/updater.go

@@ -4,11 +4,25 @@ import (
 	"context"
 	"errors"
 	"net"
+
+	"github.com/qdm12/gluetun/internal/models"
 )
 
 var ErrNotEnoughServers = errors.New("not enough servers found")
 
+type Fetcher interface {
+	FetchServers(ctx context.Context, minServers int) (servers []models.Server, err error)
+}
+
 type ParallelResolver interface {
 	Resolve(ctx context.Context, hosts []string, minToFind int) (
 		hostToIPs map[string][]net.IP, warnings []string, err error)
 }
+
+type Unzipper interface {
+	FetchAndExtract(ctx context.Context, url string) (contents map[string][]byte, err error)
+}
+
+type Warner interface {
+	Warn(s string)
+}