Create a plugin registry for multiple accessors

[sirpalee]

We need a generic plugin registry that can load up multiple accessors easily, and provide an interface that's flexible and easy enough to customize.

[chadrik]

Should the plugins just be other ArResolvers that are delegated to by the master ArResolver?

[sirpalee]

If you want the most flexible option, then yes.

However, I think we could do equally good by only exposing parts of the interface and limit the scope of each accessor. For example, I would like to generate the temporary file location on the disk using the URIResolver and making sure they are removed once the URIResolver is out of scope, so accessor don't have to worry about such problems. Same goes for encryption of passwords for database based accessors.