B-74332: externalize the capability to return feed pages in JSON

Apache Abdera's Workspace Provider, as described in the org.apache.abdera.protocol.server.Provider interface, supports plug-able filters architecture, similar to the Servlet Filters architecture. 

Atom Hopper previously leveraged this architecture to provide the capability to return feed pages in full JSON format. However, this capability is hard coded inside the code. 

The main purpose of this PR is to externalize that capability. If one needs the JSON pages, one can configure Atom Hopper appropriately.

This will be a **breaking** change for people who are relying on this JSON pages. If you want to keep this behavior, you'll have to configure Atom Hopper properly. Instructions on how to configure Atom Hopper to do such will be documented in the wiki.