add boolean plumbing column to the metadata output

[wibeasley]

Some downstream functions need a quick way to identify the plumbing variables (ie, record_id, event_name, redcap_repeating_instrument, and redcap_repeating_instance). I'm going to add a new column in the metadata tibble that indicates this, so they can be used more efficiently.

I'll add it to the non-visible function redcap_metadata_internal().