Adds CsvDataset, which both reads and parses files.

Adds CsvDataset, which both reads and parses files.
Example usage: dataset = tf.contrib.data.CsvDataset(filenames, record_defaults=record_defaults, **kwargs)
Motivation: Fusing reading and parsing is more performant and correct than the previous canonical CSV parsing flow (`dataset = tf.data.TextLineDataset(filenames).map(lambda l: tf.decode_csv(l, **kwargs))`)

Closes #19077.

PiperOrigin-RevId: 196601381