oteljava: TextMapPropagatorWrapper - add meaningful toString

[iRevive]

Before this change, the toString didn't provide any useful information:

OtelJava.autoConfigured[IO]().use { otel4s =>
  IO.println("Propagators: " + otel4s.propagators)
}

Prints:

Propagators: ContextPropagators.Default{textMapPropagator=
org.typelevel.otel4s.oteljava.context.propagation.JavaPropagatorWrappers$JTextMapPropagatorWrapper@47a8424c}