Add a SentencePiece tokenizer layer

[mattdangerw]

After landing the WordPiece tokenizer (#22) and hammering out a layerized tokenizer design, we should also support for SentencePiece tokenization. SentencePiece is both language agnostic and reversible, and will be an important addition to our tokenization offering. SentencePiece graph ops are already supported by tensorflow text.

[adhadse]

Is this TensorFlow text operation deprecated?

[mattdangerw]

The ops we would be using are here https://www.tensorflow.org/text/api_docs/python/text/SentencepieceTokenizer (not deprecated).