Resource Interface

[leogr]

Structured and Mapped Resources are different but should be identical for consumers.

For instance, ListResponse needs to use both but currently cannot.

Also, a Resource Interface could totally de-couple core and resource packages.

Since core needs to use Resource Interface it should be defined in the same package, then core could implement it for Structured and resource for Mapped.
Furthermore, third-party could use it to implement other kinds of resource.

However, the two kinds are using attributes in different ways actually I have no clear ideas how the interface should be.