make `per_filter_config` specific configs work automatically for filters

Currently filter writers need additional plumbing to allow their filter to do per vhost/route overrides. E.g.:

// Fetch the virtual host specific config.
const VHostSpecificFilterConfig* Filter::getSpecificConfig() const {
  if (!decoder_callbacks_->route() || !decoder_callbacks_->route()->routeEntry()) {
    return nullptr;
  }

  const auto* entry = decoder_callbacks_->route()->routeEntry();
  const auto* settings = entry->virtualHost().perFilterConfig("pinterest.my_filter");

  if (settings != nullptr) {
    auto* vhost_config = dynamic_cast<const VHostSpecificFilterConfig*>(settings);
    return vhost_config;
  }

  return nullptr;
}

and then you have to merge that, with the config object you got from the Filter Factory. It's not that much logic, but it's a bit of hassle to a) do it for every filter and b) remember that you have to do it.

Thoughts on providing some plumbing to avoid repeating the above pattern (or thoughts on the general future of per vhost/route config overrides)?

cc: @mattklein123 @htuch @fishcakez