Handle ExternalStatePolicy during Iterator::Save(). This CL adds an optional external_state_policy attr to the SerializeIterator op which is by default set to FAIL thereby keeping the behavior the same as before. Now users can override it if they want something else. This CL also exposes this option in the CheckpointInputPipelineHook.

PiperOrigin-RevId: 286314588
Change-Id: I4f8eae88f42c74b168348afca8b295eb37d845dc