Get the client-side web resource for storage plugins from the storage object itself

[exarkun]

Storage plugins can contribute to the web interface on client nodes. They do this by implementing IFoolscapStoragePlugin.get_client_resource. This is somewhat awkward to implement because it does not receive all of the same information that IFoolscapStoragePlugin.get_storage_client receives and collecting that information where get_client_resource needs to be called is challenging to implement.

Instead, move get_client_resource to a new interface which the object returned by get_storage_client may provide. Then, the plugin can operate on all of the same information to create the web resource without requiring the callers of these methods to shuffle more data around through various layers of abstraction.