ocppj logger is racy

[andig]

OCPP version:
[x] 1.6
[ ] 2.0.1

I'm submitting a ...

[x] bug report
[ ] feature request

Current behavior:

See this -race test;

==================
WARNING: DATA RACE
Write at 0x00010836e370 by goroutine 80:
  github.com/lorenzodonini/ocpp-go/ocppj.SetLogger()
      /Users/andig/go/pkg/mod/github.com/evcc-io/ocpp-go@v0.0.0-20240914122959-0d109e403130/ocppj/ocppj.go:43 +0x50
  github.com/evcc-io/evcc/charger.(*ocppTestSuite).SetupTest()
      /Users/andig/htdocs/evcc/charger/ocpp_test.go:41 +0x44
  github.com/stretchr/testify/suite.Run.func1()
      /Users/andig/go/pkg/mod/github.com/stretchr/testify@v1.9.0/suite/suite.go:192 +0x218
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.23.2/libexec/src/testing/testing.go:1690 +0x184
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.23.2/libexec/src/testing/testing.go:1743 +0x40

Previous read at 0x00010836e370 by goroutine 23:
  github.com/lorenzodonini/ocpp-go/ocppj.(*DefaultServerDispatcher).messagePump()
      /Users/andig/go/pkg/mod/github.com/evcc-io/ocpp-go@v0.0.0-20240914122959-0d109e403130/ocppj/dispatcher.go:556 +0x908
  github.com/lorenzodonini/ocpp-go/ocppj.(*DefaultServerDispatcher).Start.gowrap2()
      /Users/andig/go/pkg/mod/github.com/evcc-io/ocpp-go@v0.0.0-20240914122959-0d109e403130/ocppj/dispatcher.go:410 +0x34

SetLogger and accessing log would need to use a mutex. I'm not sure this is worth it though. If you think it is I'd be happy to open a PR.