Provide fine-grained control over what gets the Add Persistent Identifier form

[mjordan]

Currently, the DataCite DOI allows the abilty to control which content types get the "Add Persistent Identifier" form widgets, but the use case has come up where even within a content type it would be useful to provide more control. Maybe provide the ability to define specific values in a specific field? Or a Context (with its existing tags, etc. conditions) to add to the existing content-type control?