Refactor and clean up the Access (download) API

[landreev]

This is a "technical debt" and legacy issue.

Access is one of the oldest APIs in the application, originally implemented in the context of DVN 3. There is a lot in the code that is archaic and cumbersome and hard to work with. Prime examples:

• Unlike most of our other APIs, file downloads need to serve potentially massive amounts of data. So the content cannot be returned as a buffer or a string, and needs to be streamed. To implement streaming, in most places we are still relying on the code written for early versions of Jersey REST library; that requires providing standalone MessageBodyWriter classes (DownloadInstance.java, DownloadInstanceWriter.java etc.) It would be cleaner to switch to using javax.ws.rs.core.StreamingOutput (/api/access/datafiles, download-multiple-files-zipped, a newer API, uses it already).
• There's a system of OptionalAccessService classes, meant to support extra download options - extra formats/conversions and such. It was meant to be dynamic/possibly plugin-driven. It became hard-coded in the original Dataverse 4.0. And by now it only makes the code more confusing and difficult to work with.

[landreev]

(if we already have open issues for this matter, this issue should supersede them)