Define an interface for all the extra public methods implemented by DrupalApcCache

Since the DrupalApcCache class has more public methods than those defined in DrupalCacheInterface, it should implement an interface which defines the "extra" methods.

In this way, any class extending DrupalApcCache would know which methods are required to implement, or a module that could benefit of the "extra" methods could verify the cache back-end object is the correct class, and work with any module that extends DrupalApcCache.