Package JSON manifest should support generic attributes

I like the idea of the JSON manifest format -- it allows us to create backends for a generic packaging frontends.

I would like to adopt this for `pkg_rpm` at some point, but in order to do this, the manifest would need to support a generic "attributes" format to support file tags (like `%config`). This will also be needed for supporting packages in Windows.

So, instead of separate `mode`, `user`, and `group` fields, perhaps we should have an `attributes` object that can contain arbitrary information, mirroring `pkg_attributes` in the `pkg_filegroup` framework?