allow specification of serialization type for exported RDF

[jmartin-sul]

presently, the retrieval function has a param that defaults to ld+json, but there's no facility to override that in the calling functions, and there's nothing like class level configuration (because it's just loose functions at the moment).

spawned by #1

[michelleif]

if this is for the user to specify, then out of scope for this work cycle.

[jmartin-sul]

fwiw, it'd be pretty easy to allow whoever is running export to specify the serialization format, as long as it's something trellis supports (because we just have to tell trellis which format to use for serving the requested content).

[NancyJean]

1. Can you please put file extensions on the exported data?
2. The LC BF2MARC conversion is now available; however it is looking for rdf/xml as the input format. For most people, the easiest way to review that conversion is with MarcEdit because it requires no programming ability, but again, it requires rdf/xml. Therefore, we need to be able to export our data in a choice of serializations, but most urgently in rdf/xml.
3. BF2MARC conversion requires the merging of BF Work and Instance data to create a MARC record (where the data is intertwined). In order to do that, we need the option to export our Sinopia data as a single ref/xml file, rather than a bunch of individual, discrete files.
4. It would be useful to be able to specify what we want to export, e.g., only Works and Instances, not Items and AdminMetadata. Currently, we have no way to tell what kind of resource a single json file represents, which is annoying since you have to open each one individually. At this point, we don't know how or if the BF2MARC conversion will process AdminMetadata and Items, so the ability to include/exclude certain resource types from export would be useful.
   Thanks!