bids: Make sure we have metadata (such as acq_time) from _scans file extracted

[yarikoptic]

Something to keep in mind:

• if not anonymized (e.g. we do not do that in heudiconv yet) dates might leak sensitive information. So we might want to provide ad-hoc handling here splitting date from time and having two dedicated fields (acq_time:date, acq_time:time)?