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/utils/nofetcher.go

@@ -0,0 +1,26 @@
+package utils
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type NoFetcher struct {
+	providerName string
+}
+
+func NewNoFetcher(providerName string) *NoFetcher {
+	return &NoFetcher{
+		providerName: providerName,
+	}
+}
+
+var ErrFetcherNotSupported = errors.New("fetching of servers is not supported")
+
+func (n *NoFetcher) FetchServers(ctx context.Context, minServers int) (
+	servers []models.Server, err error) {
+	return nil, fmt.Errorf("%w: for %s", ErrFetcherNotSupported, n.providerName)
+}