[tf.data] Add a `Structure` class that represents output_{types,shapes,classes}.

This change adds the skeleton implementation of `Structure` for
tensors, sparse tensors, and nests. Future changes will (i) add
structure representations for `Dataset` and `Optional`, (ii) add
support for registering custom types, and (iii) switch
`Dataset`, `Iterator`, and `Optional` to use `Structure` internally.

PiperOrigin-RevId: 209281967