Maint: refactor VPN configuration structure

- Paves the way for Wireguard
- VPN struct contains Type, Openvpn and Provider configurations
- OpenVPN specific options (e.g. client key) moved from Provider to Openvpn configuration struct
- Move Provider configuration from OpenVPN configuration to VPN
- HTTP control server returns only openvpn settings (not provider settings)

internal/openvpn/state/openvpn.go

@@ -0,0 +1,43 @@
+package state
+
+import (
+	"context"
+	"reflect"
+
+	"github.com/qdm12/gluetun/internal/configuration"
+	"github.com/qdm12/gluetun/internal/constants"
+)
+
+type SettingsGetSetter interface {
+	GetSettings() (openvpn configuration.OpenVPN,
+		provider configuration.Provider)
+	SetSettings(ctx context.Context, openvpn configuration.OpenVPN,
+		provider configuration.Provider) (outcome string)
+}
+
+func (s *State) GetSettings() (openvpn configuration.OpenVPN,
+	provider configuration.Provider) {
+	s.settingsMu.RLock()
+	openvpn = s.openvpn
+	provider = s.provider
+	s.settingsMu.RUnlock()
+	return openvpn, provider
+}
+
+func (s *State) SetSettings(ctx context.Context,
+	openvpn configuration.OpenVPN, provider configuration.Provider) (
+	outcome string) {
+	s.settingsMu.Lock()
+	settingsUnchanged := reflect.DeepEqual(s.openvpn, openvpn) &&
+		reflect.DeepEqual(s.provider, provider)
+	if settingsUnchanged {
+		s.settingsMu.Unlock()
+		return "settings left unchanged"
+	}
+	s.openvpn = openvpn
+	s.provider = provider
+	s.settingsMu.Unlock()
+	_, _ = s.statusApplier.ApplyStatus(ctx, constants.Stopped)
+	outcome, _ = s.statusApplier.ApplyStatus(ctx, constants.Running)
+	return outcome
+}