add "dmrecord" ID to metadata export

[saracarl]

@benwbrum to confirm the field name. It's the contentdm (cdm?) record id from an import. This one? https://github.com/benwbrum/fromthepage/blob/fc2575286a46b518ac184391de48742ee1c4c522/lib/contentdm_translator.rb#L155

[benwbrum]

We can calculate a dmrecord from the at_id of a canvas, and do so already in the ContentDM translator.

So you'll want to test whether a work was imported from IIIF (so work.sc_manifest should be true), and then get the sc_manifest.at_id.

Then test that at_id to see whether or not it was a CONTENTdm import using this method

Then get the dmrecord by calling this method (which you will need to move to the public portion of the module)

Then add the column and fill it with the dmrecord!