[tf.data] Optimization checkpointing improvements.

This CL:
- changes the `OptimizeDataset` checkpointing logic to checkpoint the optimized dataset (as opposed to the original dataset + the optimizations, re-running optimization every time a checkpoint is restored)
- replaces `OpKernelContext` with newly introduced `SerializationContext` in the signature of `AsGraphDefInternal` to reduce the scope of the context and also simplify the logic for overriding the `FunctionLibraryDefinition` when optimizations take place

PiperOrigin-RevId: 208282562