Support append operation

This is listed as a TODO [here](https://github.com/google/array_record/blob/d73af3907f05dd3bff5ebf42e7f9f2b29e026455/cpp/array_record_reader.h#L30). Any chance the interface could be extended to support appends?

Start with cpp/array_record_reader.h at the TODO linked in the issue and trace the existing reader interface and its related implementations. Define the scope of append support from those interfaces, then verify that the resulting API can perform appends without breaking existing read behavior.

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