[SDK] Tracer provider shutdown blocks in-definitively

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.
