Fallback to container duration in approximate mode

[mollyxu]

Fallback to container duration in approximate mode when stream duration is unavailable

[scotts]

Rather than assigning the container's duration to the stream's duration, I think we should do something like:

if (!streamMetadata.durationSectonsFromHeader.has_value()) {
  return containerMetadata_.durationSecondsFromHeader;
}

I think that keeps it clear where we're getting our information from. So far, we've kept that explicit in our existing metadata fields.

One wrinkle here is that the metadata we report to users in Python is VideoStreamMetadata. That is, it's explicitly the stream metadata. We don't have a hierarchically clean way to add the container metadata there. So while it's kinda odd, maybe we should add a field for duration_seconds_from_container on the Python side.

To be clear, my current thinking is that:

1. In C++, we fall back to the container metadata where appropriate. This may be awkward because we don't pass around the container metadata, so we may want to add a durationSecondsFromContainer field to StreamMetadata.
2. In Python, we should add a field for duration_seconds_from_container to StreamMetadata, and we have logic to fall back to it in duration_seconds.

@NicolasHug, @Dan-Flores, I'm curious to hear your thoughts as well.