Enable ThreadDumpEndpoint on native-image

[mhalbritter]

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At the moment, the ThreadDumpEndpoint is not usable in a native image, as GraalVM doesn't implement thread dumping yet.

I've opened an issue for that: https://github.com/oracle/graal/issues/4714

When that issue is resolved, remove the

if (NativeDetector.inNativeImage()) {
  throw new ThreadDumperUnavailableException("Running in native image");
}

condition.