Maintenance: split each provider in a package

- Fix VyprVPN port
- Fix missing Auth overrides

internal/provider/mullvad/provider.go

@@ -0,0 +1,19 @@
+package mullvad
+
+import (
+	"math/rand"
+
+	"github.com/qdm12/gluetun/internal/models"
+)
+
+type Mullvad struct {
+	servers    []models.MullvadServer
+	randSource rand.Source
+}
+
+func New(servers []models.MullvadServer, randSource rand.Source) *Mullvad {
+	return &Mullvad{
+		servers:    servers,
+		randSource: randSource,
+	}
+}