Exposes metatag module's metatag_dc submodule's dublin core mappings to an OAI-PMH endpoint

A config form is available at /admin/config/services/rest/oai-pmh to set which Content Type should be included in the endpoint, and which field is used for OAI-PMH's set functionality.

Right now some very basic features are all that're implemented, but I plan on abstracting further.