Add AnonymousIteratorHandleOp for non-shared Iterator resources

Fixes Iterator cleanup when executing eagerly. DestroyResourceOp will now remove the last reference from the Iterator resource when it runs (after the last Python reference to an EagerIterator is removed).

Previously EagerIterator used IteratorHandleOp to create resource handles, which used one kernel per (unique) shared name since the shared name was an attribute. These kernels each held a reference to their resource, which kept it alive indefinitely.

Fixes #19499.

PiperOrigin-RevId: 198417997