add CoVoST2
@patrickvonplaten 
I removed the mp3 files, dummy_data is much smaller now!
This PR adds the CoVoST2 dataset for speech translation and ASR.
https://github.com/facebookresearch/covost#covost-2

The dataset requires manual download as the download page requests an email address and the URLs are temporary.

The dummy data is a bit bigger because of the mp3 files and 36 configs.