Add Librispeech ASR
This PR adds the librispeech asr dataset: https://www.tensorflow.org/datasets/catalog/librispeech

There are 2 configs: "clean" and "other" whereas there are two "train" datasets for "clean", hence the name "train.100" and "train.360".

As suggested by @lhoestq, due to the enormous size of the dataset in `.arrow` format, the speech files are not directly prepared to a float32-array, but instead just the path to the array file is stored.