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/custom/provider.go

@@ -3,18 +3,21 @@ package custom
 import (
 	"github.com/qdm12/gluetun/internal/constants/providers"
 	"github.com/qdm12/gluetun/internal/openvpn/extract"
+	"github.com/qdm12/gluetun/internal/provider/common"
 	"github.com/qdm12/gluetun/internal/provider/utils"
 )
 
 type Provider struct {
 	extractor extract.Interface
 	utils.NoPortForwarder
+	common.Fetcher
 }
 
 func New() *Provider {
 	return &Provider{
 		extractor:       extract.New(),
 		NoPortForwarder: utils.NewNoPortForwarding(providers.Custom),
+		Fetcher:         utils.NewNoFetcher(providers.Custom),
 	}
 }