Maint: healthcheck package interface rework

- return concrete struct type
- Add compilation checks for implementations

internal/healthcheck/openvpn.go

@@ -5,9 +5,16 @@ import (
 	"time"
 
 	"github.com/qdm12/gluetun/internal/constants"
+	"github.com/qdm12/gluetun/internal/openvpn"
 )
 
-func (s *server) onUnhealthyOpenvpn(ctx context.Context) {
+type openvpnHealth struct {
+	looper       openvpn.Looper
+	healthyWait  time.Duration
+	healthyTimer *time.Timer
+}
+
+func (s *Server) onUnhealthyOpenvpn(ctx context.Context) {
 	s.logger.Info("program has been unhealthy for " +
 		s.openvpn.healthyWait.String() + ": restarting OpenVPN")
 	_, _ = s.openvpn.looper.ApplyStatus(ctx, constants.Stopped)