[SequentialWriter] Allow directory to exist as long as there is no metadata.yaml

[oysstu]

https://github.com/ros2/rosbag2/blob/5062b6e7caaa04eeeaf2e9546df737a814d5533b/rosbag2_cpp/src/rosbag2_cpp/writers/sequential_writer.cpp#L117C1-L130C4

Currently SequentialWriter will throw an exception if the target logging directory already exists. Would you be open to a PR to make this a bit more permissive, such as only throwing an exception if a metadata.yaml file exists?

I have some auxiliary log files that would be nice to have in the same directory and permitting this would remove the need to ensure that the rosbag logger always starts first.