feat(server): patch VPN settings

- `PUT` at `/v1/vpn/settings`
- Undocumented, experimental for now

internal/server/interfaces.go

@@ -12,6 +12,7 @@ type VPNLooper interface {
 	ApplyStatus(ctx context.Context, status models.LoopStatus) (
 		outcome string, err error)
 	GetSettings() (settings settings.VPN)
+	SetSettings(ctx context.Context, settings settings.VPN) (outcome string)
 }
 
 type DNSLoop interface {
@@ -27,3 +28,7 @@ type PortForwardedGetter interface {
 type PublicIPLoop interface {
 	GetData() (data models.PublicIP)
 }
+
+type Storage interface {
+	GetFilterChoices(provider string) models.FilterChoices
+}