[SDK] Tracer provider shutdown blocks in-definitively

[marcalff]

The following code, in file sdk/src/trace/tracer_provider.cc:

bool TracerProvider::Shutdown() noexcept
{
  return context_->Shutdown();
}

does not take a timeout parameter.

TracerProvider::Shutdown() will block in-definitively if unsent spans are present.

This prevents an application to shutdown opentelemetry-cpp.